Error help

So i updating a PocketMine plugin called Duels, i got this error when i type create command and it said:
TypeError: "vixikhd\duels\arena\Arena::__construct(): Argument #1 ($plugin) must be of type vixikhd\duels\Duels, null given, called in C:\Users\Dotuananh\Desktop\pmmp lib\plugins\Duels\src\vixikhd\duels\commands\DuelsCommand.php on line 99" (EXCEPTION) in "plugins/Duels/src/vixikhd/duels/arena/Arena" at line 116
And here is my code on both DuelsCommand.php and Arena.php:
case "create":
if (!$sender->hasPermission("duels.cmd.create")) {
$sender->sendMessage("§cYou have not permissions to use this command!");
break;
}
if (!isset($args[1])) {
$sender->sendMessage("§cUsage: §7/sw create <arenaName>");
break;
}
if (isset($this->plugin->arenas[$args[1]])) {
$sender->sendMessage("§c> Arena $args[1] already exists!");
break;
}
$this->plugin->arenas[$args[1]] = new Arena($this->plugin, []); This thing is line 99
$sender->sendMessage("§a> Arena $args[1] created!");
break;
Now is from Arena.php:
public function __construct(Duels $plugin, array $arenaFileData)That code is on line 66
{
$this->plugin = $plugin;
$this->data = $arenaFileData;
$this->setup = !$this->enable(false);

$this->plugin->getScheduler()->scheduleRepeatingTask($this->scheduler = new ArenaScheduler($this), 20);

if ($this->setup) {
if (empty($this->data)) {
$this->createBasicData();
$this->plugin->getLogger()->error("Could not load arena {$this->data["level"]}");
} else {
$this->plugin->getLogger()->error("Could not load arena {$this->data["level"]}, complete setup.");
}
} else {
$this->loadArena();
}
}

 

Please surround your code with code blocks.
Eg:
[ PHP] code here.. [ /PHP]

 

Check what's $this->plugin variable.