Basically i set everything to false in the config to try to let customizable options exist and i put inside the template folder a data folder in hopes to inject a datapack into the saves but i was lead with a crash. You could probally duplicate with anything like leaving everything false and injecting 1 region file into the save or the level.dat

Crash: https://paste.feed-the-beast.com/view/9f8db6c6
Version: Only 1.12.1-7.0.1
Other Mods: Nothing other then forge with lockdown.
Steps to Duplicate:
1: Open Config File
2: Change all the settings to false to restore the custom world GUI
3: inject any folder inside template, like data
4: create a world
5: crash