Kept placing blocks mid air and destroying them in creative mode tons of times in quick succession, eventually game crashed with the exact same log from the related issue.
After looking around further it looks like the array is cleared. It looks like an array.clear is called while the stream is busy, which results in an empty array, hence it becomes null.
We could keep it like this with null check in the stream and it should work fine, I could also change it so the array won't be cleared if it's still busy and remove the null check, or add it on top of the null check. Not sure what the right course of action would be here.
Thanks for looking into this -- hopefully it gets noticed, since being able to use MobSpawnESP without worrying about it crashing would make my life 10x easier.