The null tile or object is still a bit of a mystery, because I don't know what it is. Not even with a debug routine I made just to inspect tile's contents. The look routine is actually seeing it as null terrain, but the debug routine shows it's a real terrain tile. So I have to check out how the look routine is determining it's a null terrain.
Doorplace search is not working currently. The reason is that the search routine fails without mask data. To fix that I'm going to rewrite the search with mask tiles. But for that I need room masks that indicate the facing of the room, so each four main directions get a mask type. I've find this useful in Kaduria, because once you have set the direction to a wall it's easy to find it later.
I'm in a process of rewriting the room creation itself, because room has to use a special routine to determine the facing directions while it's creating the room.