Hello Chris528,
Without knowing what edits you’ve made to the editor’s source, there is no way for us to know if your changes may of caused this crash. If Visual Studio is asserting at a particular line which is checking a value, I would suggest debugging the project and attempting to get the crash again.
Once you crash, select the “Break” option and take a look at the Callstack tab at the bottom right. Ensure that it is the same as before and then look at the check itself. We know that the check is seeing if Pos is NOT INDEX_NONE. In this case, it was so the editor asserted. Next, take a look at the Local tab at the bottom right and look for that Pos variable. There should be information here to show what asset it belonged to so you can have an idea what might be causing this problem.
Also, when you mention that it crashes “when i close the game”, do you mean that you’re ending a PIE session, closing a Standalone version of the game, or closing the project itself?