Issue wasn't present in 21w10a, first appeared 21w11a

When creating a custom dimension with a datapack, if structure generation is enabled it will cause any existing chunks to become either null or minecraft:ocean AFTER closing and re-opening the singleplayer world. This lags and sometimes crashes the game.

Doesn't seem consistent, in some cases flying out to chunks that are newly generated, then flying back to load old chunks can cause a crash.

Tested World Seed: -694360200004370268

To reproduce: Load the attached datapack into a new world
Run `execute in rdims:limbo run tp @s ~ ~ ~`
Once terrain is generated, exit to main menu and load the world again.

Note: Removed ~350,000 lines from latest.log, all were identical to
[11:50:29] [Render thread/WARN]: Received invalid biome id: -1

crash-2021-04-01_11.25.24-server.txt was not caused by this specific datapack, but another that used custom structures in almost identical dimension generation files