How to reproduce:
1. Open the attached "1309537_repro.zip" project
2. Go to Window -> Animation -> Animation Graph
3. Observe the Animation Graph window

Expected result: UI is shown in the Animation Graph window
Actual result: Animation Graph window is empty

Reproducible with: 1.0.0-preview.13 (2020.2.6f1)
Could not test with: 1.0.0-preview.14 (Whole Editor's windows are black), 2021.1.0b8 (Due to errors from com.unity.ugui package), 2021.2.0a6 (Due to errors from the UI Toolkit package)

Note:
- The issue is only reproducible on the Animation Graph window, which is part of the Animation package