LevelledWorld

1.0.1

Bug fix: a crash in the Nether with mods that generate chunks in the background.

• The crash. With Distant Horizons (or another mod that generates terrain on its own threads), the game could crash with "Exception ticking world" and a ConcurrentModificationException in LevelledWorld. It happened mostly in the Nether, and often again each time the world was loaded there.
• Why. When a bastion is generated, its piglins and piglin brutes are placed into the world. Those mods do this on a background thread, and LevelledWorld tried to give the new mobs their level right there, at the same moment as it levelled the mobs that normally spawn around you on the main thread. The two clashed. The Nether is full of bastions and spawns mobs all the time, so it was hit far more than the Overworld.
• The fix. LevelledWorld now only levels mobs on the main thread. Mobs placed in the background get their level as soon as their chunk is loaded, the same way as the mobs that vanilla world generation places.

No world damage: the crash never saved anything broken, so a world that crashed loads normally with 1.0.1.