To reproduce:
1. Create new project.
2. In hierarchy create Cube. (or any other object)
3. Add "Playable Director" component.
4. Open Timeline window, and Create Timeline asset.
5. Delete Cube in hierarchy.
6. Undo.

Expected result: No error.
Actual result: Error appears:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Timeline.TimelineWindow+TimelineState.RebindAnimators () (at C:/buildslave/unity/build/Extensions/Timeline/Editor/Window/TimelineState.cs:978)
UnityEditor.Timeline.TimelineWindow.OnUndoRedo () (at C:/buildslave/unity/build/Extensions/Timeline/Editor/Window/TimelineWindow_EditorCallbacks.cs:107)
UnityEditor.Undo.Internal_CallUndoRedoPerformed () (at C:/buildslave/unity/build/artifacts/generated/common/editor/UndoBindings.gen.cs:205)

Reproducible: 2017.1.0p1, 2017.2.0b4, 2017.3.0a1

Notes:
Will not reproduce if project was closed and reopened, unless a new Timeline asset is assigned to the PlayableDirector.