Sorry to piggyback on this issue, but I've noticed this only happens on Windows, unless you Disable fullscreen optimizations on the game executable's compatibility tab.
The game will still run in the background, and you can interact with it, but it will either show you a black screen, or a still image of the main menu.
Edit 2: by testing this out, I've noticed you can soft lock yourself in the level editor if you don't have a spawn point placed before play-testing. the menu bar will disappear.