How to create a widget in Wordpress. What's wrong?

Question

WordPress

There is a code:

class Penta_Royal_Nav_Menu_Widget extends WP_Widget {

        function __construct() {
            $widget_ops = array( 'description' => __('Add a custom menu to your sidebar.') );
            $control_ops = array('width' => 400, 'height' => 300);
            parent::__construct( 'nav_menu', __('Matt Royal Custom Menu'), $widget_ops, $control_ops );
        }

        function widget($args, $instance) {
            // Get menu
            $nav_menu = ! empty( $instance['nav_menu'] ) ? wp_get_nav_menu_object( $instance['nav_menu'] ) : false;

            if ( !$nav_menu )
                return;

            /** This filter is documented in wp-includes/default-widgets.php */
            $instance['title'] = apply_filters( 'widget_title', empty( $instance['title'] ) ? '' : $instance['title'], $instance, $this->id_base );
            $class = empty( $instance['class_menu'] ) ? '' : $instance['class_menu'];
            if ( !empty($instance['title']) )
                echo $args['before_title'] . $instance['title'] . $args['after_title'];

            wp_nav_menu( array( 'fallback_cb' => '', 'menu' => $nav_menu, 'container_class' => '', 'menu_class' => 'submenu ' . $class ) );

          // echo $args['after_widget'];
        }

        public function update( $new_instance, $old_instance ) {
            $instance = $old_instance;
            $instance['title'] = strip_tags( stripslashes($new_instance['title']) );
            $instance['class_menu'] = strip_tags( $new_instance['class_menu'] );
            $instance['nav_menu'] = (int) $new_instance['nav_menu'];
            return $instance;
        }

        function form( $instance ) {

            $title = isset( $instance['title'] ) ? $instance['title'] : '';
            $class = isset( $instance['class_menu'] ) ? $instance['class_menu'] : '';
            $nav_menu = isset( $instance['nav_menu'] ) ? $instance['nav_menu'] : '';

            // Get menus
            $menus = wp_get_nav_menus( array( 'orderby' => 'name' ) );

            // If no menus exists, direct the user to go and create some.
            if ( !$menus ) {
                echo '<p>'. sprintf( __('No menus have been created yet. <a href="%s">Create some</a>.'), admin_url('nav-menus.php') ) .'</p>';
                return;
            }
            ?>
            <p>
                <label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:') ?></label>
                <input type="text" class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" value="<?php echo $title; ?>" />
            </p>
            <p>
                <label for="<?php echo $this->get_field_id('class_menu'); ?>"><?php _e( 'Class menu:' ); ?></label>
                <input type="text" value="<?php echo $class; ?>" name="<?php echo $this->get_field_name('class_menu'); ?>" id="<?php echo $this->get_field_id('class_menu'); ?>" class="widefat" />
            </p>
            <p>
                <label for="<?php echo $this->get_field_id('nav_menu'); ?>"><?php _e('Select Menu:'); ?></label>
                <select id="<?php echo $this->get_field_id('nav_menu'); ?>" name="<?php echo $this->get_field_name('nav_menu'); ?>">
                    <option value="0"><?php _e( '&mdash; Select &mdash;' ) ?></option>
            <?php
                foreach ( $menus as $menu ) {
                    echo '<option value="' . $menu->term_id . '"'
                        . selected( $nav_menu, $menu->term_id, false )
                        . '>'. esc_html( $menu->name ) . '</option>';
                }
            ?>
                </select>
            </p>
            <?php
        }
    }

add_action('widgets_init', create_function('', 'register_widget("Penta_Royal_Nav_Menu_Widget");'));

What is the problem, why is the class_menu field not saved and updated?
The code is located in function.php.
wordpress version 4.3.1