Error metode insert oop php

Saya mengikuti yang ada di video php oop bagian membuat metode insert, namun mendapatkan error:

Fatal error: Uncaught Error: Call to a member function insert() on null in C:\MAMP\htdocs\auth\classes\User.php:14 Stack trace: #0 C:\MAMP\htdocs\auth\register.php(9): User->register_user(Array) #1 {main} thrown in C:\MAMP\htdocs\auth\classes\User.php on line 14

<?php

    class User{

        private $_db;

        public function __construct()
        {
            $this->_db = Database::getInstance();
        }

        public function register_user($fields = array()){
            var_dump($fields);
            if($this->_db->insert('users', $fields)) return true;
            else return false;
        }

    }

?>

public function insert($table, $fields = array()){

        //mengambil kolom
        $column = implode(",", array_keys($fields));

        //mengambil nilai
        $valueArrays = array();
        $i = 0;
        foreach ($fields as $key => $value) {
            if(is_int($value)){
                $valueArrays[$i] = $value;
            }
            else{
                $valueArrays[$i] = "'" . $value . "'";
            }
            $i++;
        }

        $values = implode(",", $valueArrays);

        $query = "INSERT INTO $table ($column) VALUES ($values)";

        if($this->mysqli->query($query)) return true;
        else return false;
    }