PlayerRespawnEvent: harden setRespawnPosition()

apparently plugins like to pass around positions which have null worlds, which aside from being quite stupid, also breaks a lot of stuff and makes it look like PM is to blame when it's just trying to make everything work the way it's supposed to ...

src/pocketmine/event/player/PlayerRespawnEvent.php

@@ -43,6 +43,9 @@ class PlayerRespawnEvent extends PlayerEvent{
 	}
 
 	public function setRespawnPosition(Position $position) : void{
+		if(!$position->isValid()){
+			throw new \InvalidArgumentException("Spawn position must reference a valid and loaded World");
+		}
 		$this->position = $position;
 	}
 }