Chunk crash after reloading by plugin

Hello! I have crash, when player enter the map, what reloaded by plugin.
Code for reload:

PHP:

//Teleport all to default world
$this->getServer()->unloadLevel($this->map);
$this->getServer()->loadLevel($this->mapName); //mapName is already saved variable


There is no error in console. But, when player join this world, PocketMine crashs:

Code:

Fatal error: Call to a member function getChunk on null in phar:///home/sw/PocketMine-MP.phar/src/pocketmine/level/Level.php on line 2765

Provider in Level is null, but why and how to fix this?

What is mapNam! You should check if it is a valid level name. Or do something like this:
var_dump($this->getServer()->getLevelByName($mapName) instanxeof Level)

It is right, because this level is load by this mapName in onEnable. But after REload map is broken

You have to still check... Show me the output of loadLevel then.