I saw this error in server.log:

16:18:03,163 ERROR [org.jbpm.designer.web.preprocessing.impl.JbpmPreprocessingUnit] (http-localhost/127.0.0.1:8080-1) Failed to setup workitems : null

I didn't add any new work items. I had only the default ones.

To reproduce:
1.) Setup BPMS 6.0.1.CR2.
2.) Create a business process with a REST task.
3.) Build&deploy project and try to run the process.
4.) I got this error BZ1082091 which should be fixed now.
5.) I returned back to designer to modify the process, I guess it was Data Assignments dialog of REST task.
6.) The error above appeared in server.log. Designer opened. The service task palette was correctly filled with default service tasks

Expected behavior:
I think the designer should either throw no error to not confuse customers or it should throw more meaningful error, which would precisely describe the existing problem.

My env:
BPMS 6.0.1.CR2
EAP 6.1.1