Steps to reproduce:
1. Open "Reduced_Repro" attached project
2. Open "Main" scene
3. Open Animator window and select "FPS_player" from hierarchy, toggle it disabled and activated to make an arbitrary change to the project
4. Select the transition in Animator window from "Idle" to "Walk"
5. In the Preview window press play
6. Ctrl + S to save the project while preview is playing
7. Unity should crash

Expected outcome: Upon saving project, unity doesn't crash, regardless of any animation previews being played.
Actual outcome: When you save the project while animation preview is running, editor crashes

Reproduced on: 5.6.0b11, 5.6.0f3, 5.6.1f1, 2017.1.0b4
Regression introduced on: 5.6.0b11
Does not reproduce on: 5.6.0b10, 5.5.3p3, 5.4.5p4