The crash can occur if a protochunk next to a FULL chunk is dropped,
and then later re-requested. If it was not persisted to disk for any
reason, it starts regeneration from scratch. At FEATURES stage, it may
try to place blocks into the adjacent LevelChunk already in the world.
The fix is to prevent this situation from even happening by pinning
protochunks directly next to FULL chunks, and preventing them from
unloading.