This is like MDL-51147, but affecting multichoice questions, not match questions.
Steps to reproduce:
- create a quiz
- add a multichoice question with 4 choices
- attempt the quiz as a student
- delete the third choice
- go to Results -> Statistics
Expected result: the report runs.
Acutal result: fatal error.
Forum thread: https://moodle.org/mod/forum/discuss.php?d=321180