MySQL Error

I am (very!) new to MySQL, so I just started coding with/in it.
My Code:

PHP:

public function getCurrentSlots($server) {
        $status = "current";
        $result = $this->database->query("SELECT * FROM slots WHERE status='$status' AND server='$server'"); // That's Line 83
        if ($result instanceof \mysqli_result) {
            $data = $result->fetch_assoc();
            $result->free();
            if (isset($data["server"]) and $data["server"] === $server) {
                unset($data["server"]);
                return $data["amount"];
            }
        }
        return null;
    }

Error when using the function:

Code:

Fatal error: Call to a member function query() on null in C:\Users\fuego\Desktop\Programmieren\Server\PocketMine-MP\plugins\MySQL\src\BukkitPlaysMC\Slots\Main.php  on line 83

Can anyone tell me what's wrong? :/

$this->database is undefined.
Also, why did you unset $data["server"]?

Idk lol and it isn't actually

Yes, $this->database is null. Try doing a var_dump() on it.