Added exception throws when a closed tile or entity attempts to schedule itself for updating

I can't believe it took so long to find what was causing these stdClass bugs.

src/pocketmine/entity/Entity.php

@@ -1377,6 +1377,9 @@ abstract class Entity extends Location implements Metadatable, EntityIds{
 	}
 
 	final public function scheduleUpdate() : void{
+		if($this->closed){
+			throw new \InvalidStateException("Cannot schedule update on garbage entity " . get_class($this));
+		}
 		$this->level->updateEntities[$this->id] = $this;
 	}