How to reproduce:
1. Open the attached project
2. Open and play Scene scene
3. Unity will crash

or
1. Create a controller with a substate machine in it that has a transition out of it to another state
2. Select substate and create Empty state with transition to Entry and Exit
3. Select the substate machine in the base layer and any transitions out of it
4. Copy
5. Create a new controller
6. Paste the substate machine into the new controller
7. Play the second controller with the substate machine that was copied in to it
8. Crash!

Not reproducible:5.0.3p3