Testing Environment randomly crashes sometimes - Using Snapshot 2020.5!

Hey! I added a "few" new biomes, items, blocks and structures to my mod, and, of course, i needed to test them in-game. IDK how to explain this, but the testing environment randomly crashes whenever i fall in creative. (turn off fly) (at least that's when it mostly happens)

Here's the crash log: https://pastebin.pl/view/15c95ed5

Oh and one more thing, this doesn't happen if i export the mod, load it in MC, and create a superflat world. It only happens in Default Generation worlds.

Thank you for reading!