1. Error when creating a world with experimental settings.
   Stages of error reproduction:

1. Open Minecraft with the current Neoforge 1.21.1 core version
2. Go to the World creation editor
3. Go to the "More" section
4. Go to the "Experiments" section
5. We enable any experiment and click "Done"
6. An pops up "Data pack validation failed"

2. Game crash when re-entering the world creation mode.
   Stages of error reproduction:

1. Open Minecraft with the current Neoforge 1.21.1 core version
2. Go to the World creation editor
3. Exit the World Creation Editor
4. Go to the World creation editor
5. The "Savin World" message will be displayed for 0.5 second
6. Emergency completion of the game
   crash-reports:

crash-2025-08-25_18.26.00-client.txt

I'm sorry, I don't speak English well.
This is my first time sending a message on Github, and I couldn't figure out how to add tag.

This error is now fixed in 2.4-b04d. Thanks for the report