As reported by François Lizotte here: https://moodle.org/mod/forum/discuss.php?d=450118

1. Create a quiz with two true-false questions.
2. Log in as a student.
3. Attempt the quiz.
4. Log in as teacher.
5. View the quiz statistics (to compute and cache the stats).
6. Delete the student's attempt.
7. Go to the Quiz -> Questions page, and remove one of the questions.
8. View the quiz statistics again.

Expected results: the report says there is no data to display now.

Acutal result: fatal error, because it tries to display the cached stats, which refers to slot 2, and the quiz now only contains one slot, and the code is not robust to this.