Call to a member function getChunk() on null

I have a hub (world named "hub") and minigame (world named "nexus")
Sometimes, when minigame finish, all player teleport to hub
write:

PHP:

Error: Call to a member function getChunk() on null
File: /src/pocketmine/level/Level
Line: 2
Type: E_ERROR

My code (reset map):

PHP:

public function EndGame(){
         foreach($this->getServer()->getOnlinePlayers() as $player){
              //update player statistic
              //update player health, inventory, spawn...
          }
           $this->UnloadMap();
           $this->LoadMap();
}

public function UnloadMap(){
         $this->getServer()->getLevelByName("nexus")->setAutoSave(false);
         $this->getServer()->unloadLevel($this->getServer()->getLevelByName("nexus"),true);
     }

     public function LoadMap(){
         $this->getServer()->loadLevel("nexus");
         $this->getLogger()->info('§2Map Nexus War reset');
     }

Where does it say getChunk in the code

The level most likely isn't loaded so it will return null.

The error is inside PocketMine source.