Trying to allocate staff to a newly-created grader will crash

[blerner]

Steps to reproduce:

• Create an assignment without a manual grader
• Have students submit to it
• Realize you'd forgotten the manual grader, so edit the assignment to add it.
• Do not click the [Create missing graders] button, but instead immediately click the [Assign graders] link instead.
• Crash

Perhaps the workflow of asking the professor to manually create the missing graders needs to be revisited? At any rate this crash should be prevented.