How to reproduce:
1. Open the attached project
2. Find Enemy1 prefab and set its animator controller to the same Enemy1 animator controller
3. Then attempt to change Enemy1 animator controller to Enemy2 animator override controller
4. Crash
5. In step 3 one can drag and drop Enemy2 animator override controller, it will also crash Unity

Workaround: If prefab is in the scene,one makes same edits and hits apply, Unity doesn't crash

Not reproducible: 5.1.4f1