Bug/Issue Making an Underground Settlement and Game Keeps Crashing

I keep encountering fatal crashes when I'm working on, or simply walking around in, my underground settlement area. It looks sort of like it's due to a mob spawning issue? I've been filling in all of the area within my apartment complex project with user-built and user placed blocks. Namely, dirt bricks for the background walls and stone brick for the physical walls. All of the apartments are user-built/placed blocks too, like mirror blocks, furniture, red brick, planks, etc. Nothing randomly generated was left in them. So perhaps the game is trying to spawn a mob where it shouldn't? This hasn't been happening at all until I started expand my underground settlement lately to a moderate size. I could be completely wrong on the reason for the crashes, too.

Any help is appreciated. I sure would like to not have to abandon my project, I'm having a blast with my subterranean apartment complex!

Thanks a lot.