SAK-26070 when the group associated with assignment was deleted, the assignment page still loads. User however won't be able to do operations on the assignment(permission error), and the For column shows the deleted group id instead of group title

git-svn-id: https://source.sakaiproject.org/svn/assignment/trunk@308192 66ffb92e-73f9-0310-93c1-f5514f145a0a