How to reproduce:
1. Create a new project
2. Create a Cube in the scene and drag it to project window so it becomes a prefab
3. Open Timeline window (Window > Timeline)
4. Create timeline for a cube
5. Delete cube from the project and from the scene
6. Create Sphere in the scene and drag it to project window so it becomes a prefab
7. Select Sphere in hierarchy

Actual result: Notice that Apply and Revert buttons are inactive. Happens when Timeline playable exist in the project
Expected result: Apply and Revert buttons should be active

Reproducible: 2017.1.1f1, 2017.2.0b10, 2017.3.0a2, 2017.3.0a6