Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the custom Editor window via the menu "Bug Reproduced > Custom Window"
3. Observe the label text, its color is coral, and the size is 30px
4. Close that window
5. Enter play mode
6. Open the "Bug Reproduced > Custom Window" again
Expected result: Label text color is coral and size is 30px
Actual result: The label is white with 12 px size, and the Error “_darkThemeStyleSheet is null“ is present
Reproducible with: 6000.0.62f1, 6000.2.10f1, 6000.3.0b7, 6000.4.0a4
Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested