How to reproduce:
1. Create a new project
2. Create new animator controller
3. Create SubStateMachine and make transition from Entry node to it
4. Double click on New SubState
5. Create Empty state
6. Go to Base layer
7. Move NewSubState a bit so it updates to default
8. Select transition between Entry and NewSubState
9. Copy it (Ctrl+C)
10. Crash

Not reproducible: 5.0.3p3