How to get online players on a map

I'm usign in a plugin:

PHP:

$onp = count($this->getServer()->getLevelByName($map)->getPlayers());


But when I start the plugin I get a error :'D

Code:

Fatal error: Call to a member function getPlayers() on null in /home/andrewbit4/Escritorio/PocketMine/plugins/PvP source/src/andrewbit4/PvP/Main.php on line 178

LEVEL NOT LOADED

PHP:

if(!$this->getServer()->isLevelLoaded(/*LevelName*/)){
$this->getServer()->loadLevel(/*LevelName*/);
}

Idk what's my error, but I'm pretty that the level is loaded...

Then your level doesn't exist or you're using the wrong name.

what does $map equal?

'arena'

Just to check.

PHP:

$map = $this->getServer()->getLevelByName('world_main');
if($map instanceof \pocketmine\level\Level){
   $playersCount = count($map->getPlayers());
}

Are you sure that's the level name? Level name !== level folder