Fix The Grading Glitch

There's currently a glitch going around whenever you try to grade something that's been written by a deleted user. 

The screen shows an error page, yet won't let you continue grading other works unless you've finished grading that essay that, as stated before, you can't grade.

A possible solution to this would be to grant the ability to skip an essay when grading, or to just automatically remove essays written by deleted users from the system.