Thanks for the files! I was able to open the project without crashing.
You mentioned above that you tried to reference the sun directly from the GameMode, and that might be problematic. In order to make sure that blueprints are reusable across multiple levels, we only allow the Level Blueprints to directly reference actors in the level. You can still reference the sun in your game mode, you just have to set it through the Level Blueprint like so:
Give that a shot, and see if you still crash!