To reproduce:

1. In a new project, go to Assets -> Create -> Animator Controller
2. Create new float property in Animator window and name it Speed
2. Create new game object
3. Add Animator Controller created in step 1
5. Select game object, created in step 2
6. In Animation window, click Create button
7. Click Add Property and click plus icon next to Animator -> Speed
8. Type any number

Expected: Unity does not crash when changing Animator's float property in Animation window
Actual: Unity crashes when changing Animator's float property in Animation window

Reproduced in 5.52f1, 5.5.2p3, 5.5.2p4
Did not reproduce in 5.6.0f2, 5.6.0f3, 2017.1.0a6
Regression introduced in 5.5.2f1