sagemath/sagenb — pull request #306 by migeruhito (2014-12-14 01:14:15+00:00)
Title: Crash when a non-existent collaborator is added to a worksheet

When a non-existent user name is added to the collaborators list of a worksheet, the notebook crashed. As a side effect, we also get a crash when all the collaborators are deleted from a previously shared worksheet (the bug affected me in this way).

The cause is similar to that pointed out at Pull request #219.