500 error on submission page after removing problem from contest

[eldering]

Your environment

Reproduced on DOMjudge 6.0DEV at UKIEPC.

Steps to reproduce

1. Create a contest and a problem in it.
2. Submit to that problem.
3. Remove the problem the contest.
4. Visit the submission page: it gives a 500 error:
   

[meisterT]

What's the expected behavior in these cases?

Warn that the action will remove submissions (if they exist) and then do it?

[nickygerritsen]

We were talking about this, but the way you add/remove problems to/from a contest currently is with a multiselect. I don't know how easy it is to add a message there...?