This is a known bug (the famous reinforcement battle ctd), but it couldn't be resolved in version 1.2 (has maybe something to do with a vannilla bug of double checking traits after battle...)
It will possibel be resolved if the move to Rome 1.5 is finished.

To avoid this type of CTD ypu can:

autocalc battles

play the battles without family members, or if you have multiple generals in one army remove them except one.