Fatal error: Call to a member function loadChunk() on null

I am trying to debug a plugin (Skywars plugin) and this error comes up every time I execute join. This is the part of the code when /sw join was successfully executed.

PHP:

                                            if($this->playersInArena[strtoupper($args[1])] < $max) {
                                                $this->gamePlayers[$name] = strtoupper($args[1]);
                                                $conf = new Config($this->getDataFolder() . strtoupper($args[1]) . ".yml", CONFIG::YAML);
                                                $world = $this->getServer()->getLevelByName($conf->get("LobbyWorld"));
                                                $x = $conf->get("LobbyX");
                                                $y = $conf->get("LobbyY");
                                                $z = $conf->get("LobbyZ");
                                                $world->loadChunk($x, $z);
                                                $world->loadChunk($world->getSpawnLocation()->getX(), $world->getSpawnLocation()->getZ());
                                                $sender->teleport($world->getSpawnLocation(), 0, 0);
                                                $pos = new Vector3($x, $y, $z);
                                                $sender->getInventory()->clearAll();
                                                $sender->setGamemode(2);
                                                $sender->teleport($pos);
                                                $sender->sendMessage("> Welcome to arena '" . $conf->get("FullName") . "'!\nThe game will start soon!");
                                                ++$this->playersInArena[strtoupper($args[1])];
                                                return true;

Fatal error: Call to a member function loadChunk() on null on line 260
(Yes, I know you aren't mine readers, but this plugin is on sale by other person, so there is a limit on how much I can put up.)
I don't think there is anything wrong for me at least, and I don't quite understand the error.
EDIT: line 260 is

PHP:

                                                $world->loadChunk($x, $z);

.

Were is the line 260?

Read below.
$world->loadChunk($x, $z);

$world is not the Level object or the level doesn't exist or not loaded... Try to load / generate the level before

If the level is generated load it else generate it and then load the chunk

and what is the function to load level?

bump. please? :3

https://github.com/PocketMine/PocketMine-MP/blob/master/src/pocketmine/Server.php#L1043

thanks. but can anyone tell me what the error means?
Fatal error: Call to a member function function() on null