Service task failed after giving value to object list (nullPointerException) what can cause this error? (log in desc)

I have a problem with my process, to be honest it worked fine for months... So i think there is something else causing the the task to fail.

I have an object list business data, in the operation i set "takes value of", and create a list of these objects, at the end of the groovy script before i return the list, i write the size of the list in the log, and it is fine, other logings are done as well, it looks like everything is good. However after the size in the log this happens and my task fails: