In normal situations this should not happen and I don't know what happend.
Since you are using de Mendix DB, you should be able to inspect the built-in DB:
Check the PUBLIC.system$userrole table to find out if there is a missing name, delete any unused rows and try again :)
Hi,
Try to remove the records which are with empty name fields in database from System.UserRole object. Run app and check once.
Hope it helps you