Need help with another plugin

So I'm working on a plugin that lets you ban players from worlds. And in the plugin I have a function that checks if the player is banned in the world. Code for that function is in a spoiler below:

Spoiler

Code:

public function checkBan(Level $level, Player $player){
        $file = new Config($this->getDataFolder()."Levels/".$this->getServer()->getLevel($level)->getName().".yml");
        if($file->get($player->getName()) !== null){
            if($file->get($player->getName()) == "Banned"){
                return true;
            }else{
                return false;
            }
        }else{
            if($this->getServer()->getPlayer($player) instanceof Player){
                $file->set($player->getName(), "Allowed");
                return false;
            }else{
                return false;
            }
        }
    }

So when someone uses the command to ban the player from a world, I get this error message:
Unhandled exception executing command 'worldban RxThunderHD world' in worldban: Call to a member function getName() on null
[17:44:52] [Server thread/CRITICAL]: Error: "Call to a member function getName() on null" (EXCEPTION) in "/PerWorldBan_v0.0.1/src/Blubberboy333/PerWorldBan/Main" at line 29

Could anyone please tell me why it is doing this and how to fix this? Thanks in advance

Not sure if this is the issue but:

Code:

}else{
if($this->getServer()->getPlayer($player) instanceof Player){
$file->set($player->getName(), "Allowed");

$player was already a Player instance, so

Code:

$this->getServer()->getPlayer($player)

returns null, so the next line would have errors.

And you don't have to check if it's an instance of Player; it has to be because of this:

public function checkBan(Level $level, Player $player){