Error of UnloadLevel

PocketMine-MP Crash Dump Wed Nov 25 15:48:59 UTC 2015

Error: Call to a member function getName() on null
File: /src/pocketmine/level/Level__32bit
Line: 2734
Type: E_ERROR

Code:
[2725] return (int) $this->time;
[2726] }
[2727]
[2728] /**
[2729] * Returns the Level name
[2730] *
[2731] * @return string
[2732] */
[2733] public function getName(){
[2734] return $this->provider->getName();
[2735] }
[2736]
[2737] /**
[2738] * Returns the Level folder name
[2739] *
[2740] * @return string
[2741] */
[2742] public function getFolderName(){
[2743] return $this->folderName;
[2744] }

Backtrace:
#0 (): pocketmine\Server->crashDump(boolean)






Error, when i try unload level
My code:
$levelname = $this->getServer()->getLevelByName("worlder");
$check = count($this->getServer()->getLevelByName("worlder")->getPlayers());
if($check == 0){
$this->reseting($levelname);
/*$lvl = $this->getServer()->getLevelByName("worlder");
$this->getServer()->unloadLevel($lvl);*/
}
}
}
}
}
}
}
//==================================
public function reseting(Level $level){
$this->getServer()->unloadLevel($level);
}

You made (?) those mistakes:

1) Firstly, onEnable(), add ...loadLevel("worlder");
2) Your world isn't loaded => You can't use getLevelByName();
3) It can also crash when you try to unload Level which is not loaded

3) hmm.....

World has been loaded by code.
loadLevel in OnEnable

do you have create this code

Yes

you can use this plugin to load and unload worlds
https://forums.pocketmine.net/plugins/manyworlds.1042/