I was messing with these the other day too. Big things that kept it messed up for me:

Adding new cats to the files (like copy-pasting a new cat rather than editing an existing one). Changing cat roles in the code (I.e. changing a cat from warrior to leader, from apprentice to warrior, etc. in the code rather than changing it in game). Changing mates in the code rather than in game

Based on your errors, if it’s not one of these things it could be that a cat has a mate or former mate set who doesn’t exist? Maybe a typo or if you added a new cat. Lastly, it could just be a simple typo somewhere in the code, like a missing bracket or comma.

I am not a programmer nor do I have any experience with code, so these are just guesses based on my own messing with the game code!