Got a Whopsie error message after the REF's turn ended. The attached save should reproduce it by just ending the turn. Log file also attached.
It's probably unrelated but in case it isn't, the game also seems to be stuck. It's been several turns since I reconquered all my colonies and erased the REF's army but it still hasn't withdrawn from the new world.
Thanks for reporting! Both reported issues should now be fixed in our development build.