Steps to reproduce:
1) Open Unity Hub and create a new 3d template project
2) Reset Layout to Default
3) Open Player Settings and dock this window next to the Inspector window
4) Enter the Play mode
Note: Player Settings window will disappear (check attached video)

Expected result: Docked window should not disappear after entering Play mode

Reproduced with:
2019.3.0a8

Not reproduced with:
2019.3.0a6, 2019.3.0a7

Regression since:
2019.3.0a8