For anyone who was affected by the save bug that I caused the game keeps 2 backups and you can recover your file that way (saves can be found in %appdata%\Roaming\CassetteBeasts\ just remove the 1 from filex.json.gz.gcpf1 where x represents your save slot and your save should be ok).

I sincerely apologize for not doing a more thorough test with the mod in isolation. However, the issue should now be resolved and saves should no longer become corrupt when using this mod. If any other crashes/issues occur please let me know with as much detail as possible so that I can better troubleshoot and find the error.
Also just a reminder the mod is still in an Alpha stage based on the number of monsters that have been added and the little outside testing that has occurred.