I have stumbled on this in the Shortcut Manager conflict window, but Looking at the code, it probably happens to any modal window that calls into EditorWindow.Close.

Steps to reproduce:
- Open Shortcut Manager and create a conflict
- For example, bind "Main Menu/Assets/Clear Shader Cache" and "Main Menu/Assets/Create/Animation" both to Ctrl+J
- Close the Shortcut Manager window
- Press Ctrl+J
- A Short Conflict modal window opens up
- Click cancel
- Crash!

The crash happens in the ContainerWindow destructor.

I'm pretty sure this used to work on 2018.x, because this flow was tested during the development of the Shortcut manager

Reproducible with: 2020.2.0a8.917
Not Reproducible with: 2020.1.0b3.3385