Congrats! Now, take it slow when adding new stuff! :happy:
Build a single area, including all the door transitions, and test it before you add your placeables. Write down every doorway that has a transition, keep track of them. so you can build a temp area for testing purposes. Don't worry so much about the transition areas you need to create, create them, but don't add anything other than the doorway connection back to the main area then test it.
Then add your placeables to improve the look of things in the main area only, and test it again.
Then add npc's or creatures, and test it again.
Now, go fill in each of the areas that you transition into and test them in the same fashion.
I know, lots of simple steps, but when you skip them, you end up with huge errors or crashes like you just experienced, and much wasted time!