Removing a user role in assigned role tab results in error 500

How to reproduce:

• In Roles page, in Assigned roles tab
• Select any role
• Click on trashcan icon to delete a user role
• Success notification toast appears but error 500 is returned (see console or network inspection tool)

This error should be catched and an error message should be displayed.

Issue should be fixed in gatekeeper API because this happens everytime.