Wordpress add_submenu_page不会使用类对象？

I'm trying to make my Wordpress submenu-page call a function from within class, that extend a parent class. The parent class expects a Object as a parameter in it's constructor.

The Object is recieved in the constructor of the parent, and it seems fine so far, but when the class later run the method page_form $this->model is null and I get Call to a member function get_primary_key() on a non-object.. Why?

I'm passing a class reference to the wordpress add_submenu_page, is it something im doing wrong in this code below? Or am I structuring my code wrong in some way?

I want to have a class or method that render a form, that other classes can make use of and pass a unique object to this form. I don't need help creating the form or objects themself, but I need some guideance in how to make this inheritance, preferably object-oriented.

My Parent class:

<?php
class Backend
{
    protected $model;

    function __construct($model)
    {
        $this->model = model;
        echo $this->model->get_primary_key() // this works fine. returns 'id' as string.
    }

    function page_form()
    {
        echo $this->model->get_primary_key(); // this gives me error,  Call to a member function get_primary_key() on a non-object.
        // This function should render a form, 
        // using parameters inside $this->model, but its NULL.
    }

}

My main plugin file:

<?php

// ...

if(is_admin()) {
    add_action( 'admin_menu', 'my_plugin_menu' );
}

function my_plugin_menu() {
    add_menu_page(
        'My plugin', // page-title
        'My plugin', // label
        'manage_options', 
        'my-plugin-menu', // unique-handle
        'settings_start' // function
    );

    $bookings = new Bookings();
    add_submenu_page( 
        'my-plugin-menu', // parent unique-handle
        'Add new Booking', // page-title
        'Add new Booking', // label
        'manage_options', 
        'my-plugin-menu-add-booking', // submenu unique-handle
        array(&$bookings, 'page_form') // this is the function to call, defined in parent class.
    );

?>

My bookings class:

<?php 

use WordPress\ORM\Model\BookingModel;
class Bookings extends Backend
{
    function __construct()
    {
        parent::__construct(new BookingModel());
    } 

    // ...

}

?>

you forget the $ of $model in the constructor of Backend ?

$this->model = $model;

when adding this $, I can access $this->model in "page_form"