Alex's Caves has THIS entity mixin:

Which calls LivingEntity.getActiveEffects(). getActiveEffects() returns LivingEntity.activeEffects.values(). LivingEntity.activeEffects is null until it is initialized in the LivingEntity constructor, so a null pointer is thrown. In oter words, LivingEntity.isInWater() will now throw a null pointer if it is called before the LivingEntity constructor is called.

The Entity constructor, which is called before the LivingEntity constructor, calls entity.getEyeHeight().

Slabfishes from the mod Environmental uses their own getEyeHeight() implementation, which calls isInWater().

This causes a null pointer. To fix, you can use an accessor on activeEffects and check if it is null before doing the contents of the mixin.

team-abnormals/environmental#426