Error: Call to a member of function on null

I don't know how to fix this, so I came here (yes I'm a big noob lol).
So this is the error:

Code:

2020-03-15 [20:25:34] [Server thread/CRITICAL]: Error: "Call to a member function banPlayerCombatHacks() on null" (EXCEPTION) in "plugins/StaffMode/src/ethaniccc/StaffMode/BanMenu" at line 40

This is my code in BanMenu.php:

PHP:

<?php

namespace ethaniccc\StaffMode;

use muqsit\invmenu\InvMenu;

use pocketmine\inventory\transaction\action\SlotChangeAction;
use pocketmine\item\Item;
use pocketmine\Player;
use pocketmine\scheduler\ClosureTask;
use pocketmine\utils\TextFormat;
use pocketmine\Server;

class BanMenu{

    private $plugin;
    
    public function _construct(StaffMode $plugin){
        $this->plugin = $plugin;
    }
    
    public function openBanMenu(Player $sender, Player $who) : void{
        $menu = InvMenu::create(InvMenu::TYPE_DOUBLE_CHEST);
        $menu->readonly();
        $menu->setName(TextFormat::RED . $who->getName() . "'s profile");
        for($i = 0; $i <= 53; ++$i) {
           $menu->getInventory()->setItem($i, Item::get(Item::STAINED_GLASS_PANE, 0, 1)->setCustomName(TextFormat::RESET . " "));
           }
        $menu->getInventory()->setItem(20, Item::get(276, 0, 1)->setCustomName(TextFormat::BOLD . TextFormat::RED . "Combat Hacks"));
        $menu->setListener(function(Player $player, Item $itemClicked, Item $itemClickedWith, SlotChangeAction $action) use ($who, $menu) : bool{
            switch ($itemClicked->getId()) {
                case Item::DIAMOND_SWORD:
                    $player->removeWindow($action->getInventory());
                    $this->plugin->banPlayerCombatHacks($who); //line of error
                    return false;
                    break;
            }
        });
        $menu->send($sender);
    }
}

Can anyone help me?

 

That means that $this->plugin is null, there is probably something wrong in the part of your code where you create the BanMenu object. (new Banmenu())

 

I fixed it by making it so that instead of my BanMenu class into going back into the main class and using a function from there, I just used the code from the function and put it in there, now it doesn't appear.