Is it a bug?

[mymailmjj]

When I assign a user or potential users using spring bean,when it return null,activiti will throw Exception below:




17:33:53.927 [main] DEBUG org.activiti.engine.impl.history.HistoryManager - Current history level: AUDIT, level required: ACTIVITY
17:33:53.933 [main] ERROR org.activiti.engine.impl.interceptor.CommandContext - Error while closing command context
org.activiti.engine.ActivitiException: Expression did not resolve to a string or collection of strings
   at org.activiti.engine.impl.bpmn.behavior.UserTaskActivityBehavior.handleAssignments(UserTaskActivityBehavior.java:132) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.UserTaskActivityBehavior.execute(UserTaskActivityBehavior.java:89) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationActivityExecute.execute(AtomicOperationActivityExecute.java:45) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerStart.eventNotificationsCompleted(AtomicOperationTransitionNotifyListenerStart.java:52) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:56) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionCreateScope.execute(AtomicOperationTransitionCreateScope.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerTake.execute(AtomicOperationTransitionNotifyListenerTake.java:66) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionDestroyScope.execute(AtomicOperationTransitionDestroyScope.java:116) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerEnd.eventNotificationsCompleted(AtomicOperationTransitionNotifyListenerEnd.java:36) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:56) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.take(ExecutionEntity.java:367) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.ExclusiveGatewayActivityBehavior.leave(ExclusiveGatewayActivityBehavior.java:79) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.FlowNodeActivityBehavior.execute(FlowNodeActivityBehavior.java:36) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationActivityExecute.execute(AtomicOperationActivityExecute.java:45) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerStart.eventNotificationsCompleted(AtomicOperationTransitionNotifyListenerStart.java:52) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:56) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionCreateScope.execute(AtomicOperationTransitionCreateScope.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerTake.execute(AtomicOperationTransitionNotifyListenerTake.java:66) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionDestroyScope.execute(AtomicOperationTransitionDestroyScope.java:116) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerEnd.eventNotificationsCompleted(AtomicOperationTransitionNotifyListenerEnd.java:36) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:56) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.take(ExecutionEntity.java:367) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.BpmnActivityBehavior.performOutgoingBehavior(BpmnActivityBehavior.java:105) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.BpmnActivityBehavior.performDefaultOutgoingBehavior(BpmnActivityBehavior.java:54) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.FlowNodeActivityBehavior.leave(FlowNodeActivityBehavior.java:44) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.AbstractBpmnActivityBehavior.leave(AbstractBpmnActivityBehavior.java:47) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.UserTaskActivityBehavior.signal(UserTaskActivityBehavior.java:96) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.signal(ExecutionEntity.java:350) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.TaskEntity.complete(TaskEntity.java:160) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cmd.CompleteTaskCmd.execute(CompleteTaskCmd.java:39) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cmd.CompleteTaskCmd.execute(CompleteTaskCmd.java:24) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cmd.NeedsActiveTaskCmd.execute(NeedsActiveTaskCmd.java:61) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandInvoker.execute(CommandInvoker.java:24) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:57) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.spring.SpringTransactionInterceptor$1.doInTransaction(SpringTransactionInterceptor.java:47) [activiti-spring-5.14.jar:5.14]
   at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:131) [spring-tx-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.activiti.spring.SpringTransactionInterceptor.execute(SpringTransactionInterceptor.java:45) [activiti-spring-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:37) [activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:40) [activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:35) [activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.TaskServiceImpl.complete(TaskServiceImpl.java:168) [activiti-engine-5.14.jar:5.14]
   at com.jinhuhang.workflow.activiti.impl.listener.SkipEmptyApproverTaskListener.notify(SkipEmptyApproverTaskListener.java:238) ~[classes/:na]
   at org.activiti.engine.impl.delegate.TaskListenerInvocation.invoke(TaskListenerInvocation.java:34) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.delegate.DelegateInvocation.proceed(DelegateInvocation.java:37) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.delegate.DefaultDelegateInterceptor.handleInvocation(DefaultDelegateInterceptor.java:25) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.helper.ClassDelegate.notify(ClassDelegate.java:93) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.delegate.TaskListenerInvocation.invoke(TaskListenerInvocation.java:34) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.delegate.DelegateInvocation.proceed(DelegateInvocation.java:37) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.delegate.DefaultDelegateInterceptor.handleInvocation(DefaultDelegateInterceptor.java:25) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.TaskEntity.fireEvent(TaskEntity.java:549) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.UserTaskActivityBehavior.execute(UserTaskActivityBehavior.java:92) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationActivityExecute.execute(AtomicOperationActivityExecute.java:45) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerStart.eventNotificationsCompleted(AtomicOperationTransitionNotifyListenerStart.java:52) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:56) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionCreateScope.execute(AtomicOperationTransitionCreateScope.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerTake.execute(AtomicOperationTransitionNotifyListenerTake.java:66) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionDestroyScope.execute(AtomicOperationTransitionDestroyScope.java:116) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AtomicOperationTransitionNotifyListenerEnd.eventNotificationsCompleted(AtomicOperationTransitionNotifyListenerEnd.java:36) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:56) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.pvm.runtime.AbstractEventAtomicOperation.execute(AbstractEventAtomicOperation.java:49) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContext.performOperation(CommandContext.java:88) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperationSync(ExecutionEntity.java:532) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:527) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.take(ExecutionEntity.java:367) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.BpmnActivityBehavior.performOutgoingBehavior(BpmnActivityBehavior.java:105) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.BpmnActivityBehavior.performDefaultOutgoingBehavior(BpmnActivityBehavior.java:54) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.FlowNodeActivityBehavior.leave(FlowNodeActivityBehavior.java:44) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.AbstractBpmnActivityBehavior.leave(AbstractBpmnActivityBehavior.java:47) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.bpmn.behavior.UserTaskActivityBehavior.signal(UserTaskActivityBehavior.java:96) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.ExecutionEntity.signal(ExecutionEntity.java:350) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.persistence.entity.TaskEntity.complete(TaskEntity.java:160) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cmd.CompleteTaskCmd.execute(CompleteTaskCmd.java:39) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cmd.CompleteTaskCmd.execute(CompleteTaskCmd.java:24) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cmd.NeedsActiveTaskCmd.execute(NeedsActiveTaskCmd.java:61) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandInvoker.execute(CommandInvoker.java:24) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:57) ~[activiti-engine-5.14.jar:5.14]
   at org.activiti.spring.SpringTransactionInterceptor$1.doInTransaction(SpringTransactionInterceptor.java:47) [activiti-spring-5.14.jar:5.14]
   at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:131) [spring-tx-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.activiti.spring.SpringTransactionInterceptor.execute(SpringTransactionInterceptor.java:45) [activiti-spring-5.14.jar:5.14]
   at org.activiti.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:37) [activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:40) [activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:35) [activiti-engine-5.14.jar:5.14]
   at org.activiti.engine.impl.TaskServiceImpl.complete(TaskServiceImpl.java:172) [activiti-engine-5.14.jar:5.14]
   at com.jinhuhang.workflow.service.impl.WorkFlowManagerImpl.submitTask(WorkFlowManagerImpl.java:310) [classes/:na]
   at com.jinhuhang.workflow.service.impl.WorkFlowManagerImpl.submitTask(WorkFlowManagerImpl.java:253) [classes/:na]
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_67]
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_67]
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_67]
   at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_67]
   at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317) [spring-aop-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:183) [spring-aop-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150) [spring-aop-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:96) [spring-tx-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:260) [spring-tx-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:94) [spring-tx-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172) [spring-aop-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:91) [spring-aop-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172) [spring-aop-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204) [spring-aop-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at com.sun.proxy.$Proxy43.submitTask(Unknown Source) [na:na]
   at com.jinhuhang.workflow.dao.WorkFlowTest.submitTask(WorkFlowTest.java:91) [test-classes/:na]
   at com.jinhuhang.workflow.dao.WorkFlowTest.test5(WorkFlowTest.java:146) [test-classes/:na]
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_67]
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_67]
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_67]
   at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_67]
   at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47) [junit-4.11.jar:na]
   at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) [junit-4.11.jar:na]
   at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44) [junit-4.11.jar:na]
   at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) [junit-4.11.jar:na]
   at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:83) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:88) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238) [junit-4.11.jar:na]
   at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63) [junit-4.11.jar:na]
   at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236) [junit-4.11.jar:na]
   at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53) [junit-4.11.jar:na]
   at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229) [junit-4.11.jar:na]
   at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.junit.runners.ParentRunner.run(ParentRunner.java:309) [junit-4.11.jar:na]
   at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174) [spring-test-3.2.5.RELEASE.jar:3.2.5.RELEASE]
   at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50) [.cp/:na]
   at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) [.cp/:na]
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459) [.cp/:na]
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675) [.cp/:na]
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382) [.cp/:na]
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192) [.cp/:na]

[vasile_dirla]

Hi,
no it's not a bug, it's a feature (checked exception)

as you said: <code>value = null</code>

<code>
  if (value instanceof String) {
          List<String> candiates = extractCandidates((String) value);
          task.addCandidateUsers(candiates);
        } else if (value instanceof Collection) {
          task.addCandidateUsers((Collection) value);
        } else {
          throw new ActivitiException("Expression did not resolve to a string or collection of strings");
        }
</code>