How to reproduce:
1. Create a new empty project
2. Open Window -> Sequencing -> Timeline
3. In Hierarchy, create and select an empty GameObject
4. In Timeline window, Create and save a new timeline
5. Lock the Timeline window
6. Save the scene
7. Quit and reopen Unity
8. In Hierarchy, select "GameObject"
9. Observe Timeline window

Expected result: The "GameObject"'s timeline is visible and interactable
Actual result: Timeline is locked at a blank state, unable to unlock the timeline window

Reproducible with: 2018.4.14f1, 2019.2.15f1, 2019.3.0f1, 2020.1.0a15
Not reproducible with: 2017.4.35f1