Steps to reproduce:
1. Create a project which has new CharacterAnimation.controller
2. Open Animator window to edit CharacterAnimation.controller
3. Create new Sub-State Machine
4. Create new State inside Sub-State Machine
5. Save the scene
6. Delete State inside Sub-State Machine
7. Go up to base layer and delete Sub-State Machine
8. Undo until deleted Sub-State Machine reappears
9. Open Sub-State Machine
10. Create a new State
11. Save and close the project
12. Open the project again - it crashes.