How to reproduce:
1. Open the attached project
2. Open custom window in Window -> My Window
3. Doc it to Unity and make sure that it is separated from all other windows
4. Open MyWindow editor script
5. Create an error (example, theerror;)
6. Open Unity again and notice compilation error
7. Expand MyWindow a bit and notice that it is gone ("Failed to load")
8. Fix error in the script
9. Try to reopen My Window and notice that nothing happens
10. To make it work again one needs to select another layout

Issue introduced in: 5.3.4p4