Deleting an assigned compendium causes a blank tab

[geoidesic]

1. Create an Item compendium
2. Drag a class into it
3. Assign the class to Actor Studio Compendium Assignments in Settings
4. Delete the compendium from the game
5. Navigate to Class tab on Actor Studio
6. See blank page and console error

1. [x] Make this more robust, if a compendium is missing it should be unassigned.