How to reproduce:
1. Create animator controller with name "a"
2. Create some animation states in animator controller
3. Create AnimatorOverrideController with name "b" and set to it
animator controller "a"
4. Create empty gameObject on the scene, add Animator component to it
5. Set AnimatorOverrideController "b" to Controller field of Animator
component
6. Open Animator Window
7. Select gameObject with Animator in Hierarchy
8. Animator window does not show any parameters or states

Expected result:
Animator window shows all states of animator override controller like it was in previous versions of UnityEditor

Fix is merged to 5.2.1p1