To reproduce:
1. Open project
2. In menu bar choose Window -> Test Window.
3. Dock the Test Window next to the Hierarchy(not necessary, but makes repro easier)
4. Click on the Popup Menu and change from "Test1" to "Test2", click 'OK' in message box
5. If null reference exception is not thrown, repeat 4th step multiple times

Expected vs actual: no errors should be thrown

Reproduced: 5.6.5p1, 2017.1.3f1, 2017.2.1p3, 2017.3.0p4, 2018.1.0b6
Not reproducible: 2018.2.0a1(210ec0022b93)

We decided to close this case as won’t fix for now. After assessing it’s severity we came to the conclusion that there are currently other more pressing issues in our backlog demanding our attention than we will be able to resolve in the foreseeable future. That doesn’t necessarily mean that this bug will never be fixed, as we reassess cases like these on a regular basis.