Require assistance on Lab 5 in RTC workshop - Aspect Editor is appearing and disappearing

Hello all,

I tried to follow and setup Lab5 provided in the RTC workshop, which includes, Build on state change participant making it configurable.

I have configured everything, but in the operation behavior - follow-up action when i click on the build on state change participant. sometimes the configurable window is appearing, but the dropdown is empty.

Also I sometimes get the following error  :

An error has occurred. See error log for more details.
null argument:

I dont see any log files for this error in server logs.

Could you please help me on this.