Worlds won't load

[Spitfire421]

So I'm trying to load a pre-made demo world in the WWII guns and fun + other mod. When I go to play, it says loading, then building terrain, then i see just the background and......the mod shuts down and brings me back to the launcher. I don't understand what is going on. I've tried everything and looked through every file to see what might be causing it, but I haven't found anything. Please help.