Hi Tijs,
Thank you very much for your help.
The issue has been fixed , the problem was my main class code was using activiti.cfg.xml .As part of custom form property type, implemented into actoviti-explorer app was using activiti-custom-context.xml. so activiti.cfg.xml was overriding the changes now by making entries for custom form type into activiti.cfg.xml resolved the issue.
Thanks
Regards,
Jai