Attempted to fix a critical bug reported by multiple users, which the hoppers will randomly stop working after server restarts. Suspecting world and/or chunk changes after restart that causes the entry from the cache not to be read, therefore entry has been changed to UUID (world) /string (chunk) variable based instead of world/chunk variable based.
As this bug is really tricky to fix, this update might not work as intended. Therefore, if any of you encounters this bug again, please let me know asap.
Also fixed NullPointerException that occurs if the world was changed.