It is not straight forward to determine the root cause of this issue from the UI error as you have displayed. The error reported in the Screenshot is simply a NullReference Exception which could result from other reasons, including wrong Source code implementation.
My recommendation is to check the Stackdriver logs to understand the underlying issue. You can access the Stackdriver logs of your application using this link <https://cloud.google.com/logging/>. Please provide the errors reported in the logs to give better context to this issue observed. -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to google-appengine+unsubscr...@googlegroups.com. To post to this group, send email to google-appengine@googlegroups.com. Visit this group at https://groups.google.com/group/google-appengine. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/2b9a67c7-6b11-4f0b-810b-de3d3101b42c%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
