Reproduction steps:
1. Open the attached project “Test”
2. Open Example > Close() Example
3. Lose focus for “Close() Example” Window (click anywhere in the Editor)

Expected result: Custom Window is closed when it loses focus
Actual result: Editor crashes/error is thrown

Reproducible with: 2020.3.39f1, 2021.3.8f1, 2022.1.14f1, 2022.2.0b6, 2023.1.0a8

Reproducible on: Windows 10

Notes:
Error “NullReferenceException: Object reference not set to an instance of an object” is thrown in 2020.3.39f1.
With 2021.3 - 2023.1 streams editor crashes.

First lines of the stack trace:
0x00007ff6c63fc6e8 (Unity) GUIView::ProcessInputEvent
0x00007ff6c69b3fdd (Unity) GUIView::DoPaint
0x00007ff6c69bd58c (Unity) GUIView::RepaintAll
0x00007ff6c652c063 (Unity) Application::TickTimer
0x00007ff6c6990c7a (Unity) MainMessageLoop
0x00007ff6c6995a64 (Unity) WinMain