Coding error on grading lesson

On going to the "Grade essays" tab on the lesson, it is possible to get a coding error under certain circumstances. The error is accompanied by the following notice:

Notice: Undefined offset: 73371 in /moodle/web/mod/lesson/essay.php on line 330 Call Stack: 0.0026 968608 1. {main}() /moodle/web/mod/lesson/essay.php:0 Notice: Undefined offset: 73371 in /moodle/web/mod/lesson/essay.php on line 375 Call Stack: 0.0026 968608 1. {main}() /moodle/web/mod/lesson/essay.php:0

The circumstances seem to be

1. the lesson is limited to group members only
2. there is an essay submission from someone who is not a group member in the target grouping

To reproduce:

1. Create a lesson with "Available for group members only" unset
2. Add 2 users, "User 1" and "User 2" to the course. Login as both users and submit an assignment.
3. Log back in as admin. Create a group with only User 1 in it, and add to a grouping. Change the lesson settings to "Available for group members only" and use this grouping.
4. Go to the "Grade essays" tab. You should see the error.

(NB: This error does not appear unless there is at least one submission by someone who is in a group in the chosen grouping)

Even though the steps above describe a not particularly sensible scenario, where someone changes the target grouping after submission has started, I think (although I haven't tested this) that the same should happen if someone submits an essay assignment and then is removed from a group, which is a more reasonable scenario.