It is possible to create an assignment whose mapping doesn't exist. Therefore when you go and try and view it in the UI all you can see is a blank page with nothing on it.
Steps to reproduce.
1) Start IDM
2) Create an assignment that has a bad mappingName
3) Navigate to Manage->Assignment
4) Verify that your assignment is there with a mapping that doesn't exist.
5) Try to view it, yo will see a blank page.
Attached are images for 3) and 5) views, respectively.