When I save and exit, then come back into my dimension, before the world loads, I am greeted with the following crash:

http://pastebin.com/31NxFvwx

The crash seems to happen when the world tries to check unloaded chunks for lava for whatever reason. If I delete the dimension folder and reload it, it works perfectly fine. What is even happening?

I ended up fixing this crash by adding the following to my WorldProvider class:

@Override
public void onPlayerAdded(EntityPlayerMP p) {
int cx = ((int)p.posX) >> 4;
int cz = ((int)p.posZ) >> 4;
for(int x = cx - 1; x <= cx + 1; x++) {
	for(int z = cz - 1; z <= cz + 1; z++) {
		this.worldObj.getChunkProvider().provideChunk(x, z);
	}
}
}

which basically preloads a 3x3 chunk area centered on the player before the above crash could happen. It's a bit strange, but it works.