An issue was identified in  MDLQA-18959 where an "undefined array key" warning appears when attempting to revert grade overrides in gradebook. This seems to occur under specific conditions, depending on the error reporting settings.

Steps to reproduce:

1. Login as admin.
2. In Site administration > Development > Debugging set:
   • Debug messages: NORMAL
   • Display debug messages: Yes
3. Create a course and enrol at least one student.
4. Create at least one gradable activity in the course (e.g. Assignment)
5. Go to the Grader report in the course.
6. Enable edit mode and add a grade for the student for the activity.
7. Now, go to the Edit grade page by clicking on the 'Edit grade' option from the contextual menu next to the grade in the report.
8. In the Edit grade page, uncheck the 'Overridden' setting and notice that the feedback text area is now disabled.
9. Save changes
   • expected: the changes are saved and the grade overridde has been reverted.
   • actual: an error is shown: Undefined array key "text" in /opt/app/grade/edit/tree/grade.php on line 171