All code references are Yarn - 1.18.2 (sorry)
Carpet Crashes when Minecraft is loaded with isDevelopment being true. The reason is:
net.minecraft.Bootstrap#logMissing() called from logMissingBiomePlacementModifier() calls CommandManager.checkMissing()
which loads the CommandManager before onGameStarted() is called. This means settingsManager is null and it fails.

Carpet-Fixes has a fix for this: FxMorin/carpet-fixes@5c0780f
With MixinExtra so we don't collide with each other: FxMorin/carpet-fixes@c048edc

By simply disabling isDevelopment from loading the commandManager inside of logMissing()