I was trying to reproduce another bug where the game would stop responding (as in white out, have to end process) when I tried to build a wall up against the steep earth. I failed to reproduce that one then decided to remove the wall building I was making. It popped up with the error in the screenshot and I lost control in the game but everything seemed to still be moving. I had to alt+f4 it as control never returned.
Also there is the other bug where the game hangs when I try to end my wall at a natural earth wall but I’m sure there is more to it than that. I probably need to make a video but it’ll take a few hours to get it on my dropbox.
It seems to happen when part of the wall has been built and the hearthlings take that part down.
Another edit, I can’t seem to get it to reproduce but will add more if/when I do.