No : the Modeler is not supporting an error boundary with errorCode=null (exception when deploying).
This MUST be supported at it is explained in #ACT-953 and means : get ANY BPMN Error thown from that node / no special code trigged.
The use case I described in my previous was
1) Using Activiti Designer (Eclipse), design a .bpmn with an error boundary / error code=NULL.
2) Deploy the .bpmn it in Activiti : deployment is OK and the process is running (OK).
3) Convet the process into editable model (workspace) / step into Activiti Modeler.
4) Do nothing (!) just save the model from Activiti Modeler.
5) Try to deploy it
=> EXCEPTION / validation error.
So the validation logic is different if you upload a .bpmn and deploy it into Activiti or if you edit it within Activiti Modeler.
According the exception, it sounds like the *parser* is different / coming from uploading .bpmn or Activiti Modeler.