The following stack trace is produced when unoveriding a grade in the gradebook for a user who has a submission but no grade in an assignment when the feedback comments plugin is enabled for the assignment.

Argument 1 passed to assign_feedback_comments::text_for_gradebook() must be an instance of stdClass, boolean given, called in /git/working/moodle23/mod/assign/locallib.php on line 3356 and defined
Error code: codingerror
Stack trace:

line 397 of /lib/setuplib.php: coding_exception thrown
line 290 of /mod/assign/feedback/comments/locallib.php: call to default_error_handler()
line 3356 of /mod/assign/locallib.php: call to assign_feedback_comments->text_for_gradebook()
line 760 of /mod/assign/lib.php: call to assign->get_user_grades_for_gradebook()
line 777 of /mod/assign/lib.php: call to assign_get_user_grades()
line 1190 of /lib/gradelib.php: call to assign_update_grades()
line 1447 of /lib/grade/grade_item.php: call to grade_update_mod_grades()
line 319 of /lib/grade/grade_grade.php: call to grade_item->refresh_grades()
line 215 of /grade/edit/tree/grade.php: call to grade_grade->set_overridden()