Steps to reproduce:

1. Add a feedback activity to a course.
2. Go to "Edit questions" and add a new numeric question.
3. Click on "Edit" > "Delete question" and confirm.
4. Browser is redirected to a URL ending with "/mod/feedback/undefined".

Sometimes the question is deleted correctly, you may need to try it a few times to see the error. Once the error happens, though, all the following attempts will fail.