Steps to reproduce:
1. Set grade options to "letter grades with +/-"
2. Add an assignment or two, and go to the "Course Grade" screen (final row in the "overview")
3. Enter a "Grade overrides" using +/- values for one student
4. Delete the student that has an overridden grade
5. Go back to "Grade Options" and try to change the grade type to plain "letter grades". Click "save" at the bottom of the page
The result is an error that the grade overrides prevent changing the grade type, but returning to the override screen shows nothing because the student with the overridden grade is not shown.
See related bug
Deleted students either need to be permanently removed, completely ignored in all functions unless they are "undeleted", or still accessible through the user interface so that the data that is affecting other things can be changed.