I got this error in my game when I created a custom block that was supposed to have a return value but did not return anything because I forgot to include the "return" block for all code paths. If that is not the case for you, I would try to deactivate all the events you were working on last until the white screen problem goes away (You might of course get other errors if your game is dependent on the other events, but the goal is to get the game "running" again beyond the white screen) Then activate the events back one by one to see in which event produces the white screen problem.