Call to a member function prepare() on null in crud.class.php on line 46


I don't know why I have this error, I am trying to use PDO to call a prepare method but it gives me errors

Here is my code:

        public function create($user,$db){
        $return_array = array("success"=>true,"message"=>"");
        $create_user = new User("", "", "", "");
        //validate the user
        $validation = new Validation();
            $return_array['success'] = false;
            $return_array['message'] = $validation->get_username_criteria()."\n";
            echo $return_array['message'];
            $return_array['success'] = false;
            $return_array['message'] = $validation->get_password_criteria()."\n";
            echo $return_array['message'];
            $insert_query = "INSERT INTO 'user' ('username', 'password', 'level')VALUES (:username, :password, :level)";
            $stmt = $this->db->prepare($this->insert_query);
            $stmt->bindValue(":username", $create_user->getUsername(), PDO::PARAM_STR);
            $stmt->bindValue(":password", $create_user->getPassword(), PDO::PARAM_STR);
            $stmt->bindValue(":level", $create_user->getLevel(), PDO::PARAM_INT);
            $stmt->bindValue(":id", $id, PDO::PARAM_INT);
            echo "lalala";
            return $return_array;

            echo "lululu";
            return $return_array;


here is my connection to database:

class Database{
private $DB_USER="";
private $DB_PASS="";
private $DB_NAME="";
private $DB_HOST="";

private $db;

public function _construct(){
    $this->db = new PDO("mysql:host={$DB_HOST};dbname={$DB_NAME}", $DB_USER, $DB_PASS);

public function get_db(){
    return $this->db;

public function _destruct(){
    $this->db = null;

and here is where I call the method create():

$username = $_POST['username'];
$password = $_POST['password'];
$level = $_POST['level'];
$id = $_POST['id']; 
$user = new User($id,$username,$password,$level);
$crud_obj = new crud();
$db_obj = new Database();
    echo "Successfully registered!";

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| pdo   | php   | mysql   2017-01-06 22:01 0 Answers

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