Maximizing the docked Preview window of the following assets causes the editor to throw NullReferenceException
1. Material
2. Animation
3. Video
4. Audio

Steps to repro:
1. Create a New Project.
2. Project > Create Material > Inspector > Burgger button > Convert to Floating window
3. Dock the Preview Window > Maximize.

Actual Result:
NullReferenceException is thrown when the Preview window is maximized

Expected Result:
Preview window Maximizes and stays active without any exception being thrown

Tested with Unity Version:
2020.2.0a16, 2020.1.0b14, 2019.4.2f1,2019.3.15f1, 2018.4.24.f1.

Environment:
Windows 10 & macOS 10.14.