Stuck with GUI (1.13)

Hey!
I'm trying to update my plugin to 1.13 but I can't figure out what's the problem and why not fill my GUI with items.
Here's my code:
Inventories.java:

Code:

String asd = Vars.prefix + Vars.mainMenuHeader;
   
    @SuppressWarnings("static-access")
    public Inventory mainMenu = Bukkit.createInventory(null, 9, asd);

    @SuppressWarnings("static-access")
    public void initContents(Player p, Inventory inv) {
        if(inv == null) return;
        @SuppressWarnings("deprecation")
        ItemStack placeHolder = new ItemStack(Material.GRAY_STAINED_GLASS_PANE, 1, DyeColor.GRAY.getWoolData());
        ItemMeta im = placeHolder.getItemMeta();
        im.setDisplayName(" ");
        placeHolder.setItemMeta(im);
        for(int i = 0; i < 8; i++) {
            //inv.setItem(i, placeHolder);
        }
       
        if(inv.getName().equals(asd)) {
            if(p.hasPermission(perms.boosters_admin) || p.isOp()) {
                inv.setItem(0, new Item(Material.PAPER, "§6Boosters parancs rejtett funckiói", new String[] 
                    {
                        "§7Ezt csak az engedéllyel rendelkező emberek látják!",
                        "",
                        "§c/boosters stop - Egy már futó booster leállítása",
                        "§c/boosters give - 1 darab booster addolása",
                        "§c/boosters list - Jelenleg aktív boosterek"
                    }, 1).getItemStack());
            }
            inv.setItem(3, new Item(Material.GOLDEN_APPLE, "§9Booster aktiváló", new String[]
                {
                    "§7Itt tudsz boostereket aktiválni!"
                }, 1).getItemStack());
            inv.setItem(5, new Item(Material.DIAMOND_SWORD, "§9Booster vásárló", new String[]
                {
                    "§7Itt tudsz boostereket vásárolni!"
                }, 1).getItemStack());
            inv.setItem(8, new Item(Material.BARRIER, "§4Kilépés", new String[]{}, 1).getItemStack());
        }
    }

 

@bigflori

For debugging, it would be really useful to print all of the contents of the inventory at the end of the initContents method.

 

@knokko I figured out, for some reason it doesn't work if I use variables for the inventory name and in the if statement