I've seen two other instances of this, it appears that some building is causing the problem when you attempt to view the settlement. The last file to be accessed are related to the egyptian_stone_wall, but I've been unable to find any solution to it. Thankfully it is a very rare bug, but that doesn't help you now... I'm afraid I don't have any other advice to give than to reload an earlier save (and risk that it still happens) or start a new campaign, sorry.