Simple workaround: restart the game and all will be fine. Still, looks like a bug to me

A small test mod to demonstrate: https://www.dropbox.com/s/wd5j2b8zn5c0a ... g.zip?dl=0
- start a game
- click the button, you will get an error, click ok
- start a new game, the error will still be there/be there again, without pushing the button