Game freezes when trying to create an alien

Please help! I have tried multiple times to create an alien sim, both through adding a sim to the household through the "add occult sim" button, and through trying to change a human sim into an alien using the new feature that sits above the traits in CAS. When I try to create an alien, my game freezes. I use custom content and mods, but I have tried removing all of them and creating an alien with just the base game (and all of my packs, of course), and it's doing the same thing. If you know how to fix this please let me know! ☹️