My solution to the error: Pure virtual function being called while application was running (GIsRunning == 1)
LowLevelFatalError [File:D:\build\++UE5\Sync\Engine\Source\Runtime\Core\Private\Windows\WindowsPlatformMisc.cpp] [Line: 439]
First, in Properties > Compatibility, I set the program to run in Windows 8 compatibility mode, checked the box to run this program as administrator, and disabled fullscreen optimization. Even so, I still got the error. I deleted the two configuration folders in the game directory, created a shortcut to the game, and in Properties > Target, I wrote the following (with a space): -dx11
Doing all that solved the error. It's best to follow the steps and test each one, since I'm not sure if it was the combination of everything or just the last step that fixed it. I hope this helps someone. luck!