How to reproduce:
1. Create a new project in 2020.1.0a23
2. Move the Hierarchy window by dragging the name of the tab
3. Dock the Hierarchy tab below the Inspector window

Expected result: The Hierarchy tab is docked below the Inspector window
Actual result: The Hierarchy tab is docked below the Inspector window and a "NullReferenceException" is thrown in the Console

Reproducible with: 2020.1.0a22, 2020.1.0a23
Not reproducible with: 2017.4.37f1, 2018.4.17f1, 2019.2.21f1, 2019.3.1f1, 2020.1.0a21

Note: The issue only occurs after moving the Hierarchy tab, from an area where there are no other tabs besides the Hierarchy tab itself