Trying 1.22 which i like so far and have another crash for you.
please find attached the crash report and the saved game.
Thanks. Found the problem: Barbarians aren't able to build/possess certain units (e.g. Settlers) and this wasn't being checked before the units in a rebel city are converted. Single file fix attached. It will also prevent the rebel civ from receiving any of the old civ's unique units.