How to reproduce:
1. Create a new project and open it
2. Add any texture to the project or just create a new render texture and select it
3. In its Inspector under preview click the "3 vertical dot" button and click "Convert to Floating Window"
4. Unfocus the window by clicking somewhere else in the Editor
5. Observe the console

Expected result: Preview window is unfocused, window no longer displays anything and no errors are thrown
Actual result: NullReferenceException and GUI errors are thrown

Reproduced in: 2021.1.0a1, 2020.2.0b3
Not reproducible in: 2020.2.0b2, 2020.1.6f1, 2019.4.10f1, 2018.4.27f1