pocketmine\Server::getLevel PHP Method

    public function getLevel($levelId)
    {
        if (isset($this->levels[$levelId])) {
            return $this->levels[$levelId];
        }
        return null;
    }

 public function onCompletion(Server $server)
 {
     $level = $server->getLevel($this->levelId);
     if ($level instanceof Level and $this->hasResult()) {
         $level->chunkRequestCallback($this->chunkX, $this->chunkZ, $this->getResult());
     }
 }