taxonomy crawling for term posts

<h1>Navigating the products</h1>
    <?php
        //Get a navigation including all products
    ?>
    <ul>
        <?php
            //http://codex.wordpress.org/Template_Tags/wp_list_categories
            $args = array(
                'show_option_all'    => null,
                'orderby'            => 'name',
                'order'              => 'ASC',
                'style'              => 'list',
                'show_count'         => 0,
                'hide_empty'         => 1,
                'use_desc_for_title' => 1,
                'child_of'           => 0,
                'feed'               => null,
                'feed_type'          => null,
                'feed_image'         => null,
                'exclude'            => null,
                'exclude_tree'       => null,
                'include'            => null,
                'hierarchical'       => true,
                'title_li'           => __( 'Product categories' ),
                'show_option_none'   => __('No product categories available'),
                'number'             => null,
                'echo'               => 1,
                'depth'              => 2,
                'current_category'   => 0,
                'pad_counts'         => 0,
                'taxonomy'           => 'product_category',
                //'walker'             => 'Walker_Category'
                'walker'             => new walker_taxonomy_term_posts()
            );

            class walker_taxonomy_term_posts extends Walker_Category {

                function start_el(&$output, $category, $depth, $args) {

                    $this->category = $category;

                    parent::start_el($output, $category, $depth, $args);
                }

                function end_el(&$output, $page, $depth, $args) {
                    if ( 'list' != $args['style'] )
                        return;

                    //echo $this->category->term_id . ',';
                    echo '<pre>';
                    print_r($this);
                    echo '</pre>';

                    $posts = new WP_Query(
                        array(
                            'post_type' => 'products',
                            'tax_query' => array(
                                array(
                                    'taxonomy' => 'product_category',
                                    'field' => 'id',
                                    'terms' => $this->category->term_id
                                )
                            ),
                            'numberposts' => 3
                        )
                    );

                    if( !empty( $posts ) ) {

                        $posts_list = '<ul>';

                            while ( $posts->have_posts() ) : $posts->the_post();
                                $posts_list .= '<li><a href="' . get_permalink( $post->ID ) . '">'.get_the_title( $post->ID ).'</a></li>';
                            endwhile;

                            wp_reset_query();

                        $posts_list .= '</ul>';
                    }else{
                        $posts_list = '';
                    }

                    $output .= "{$posts_list}</li>\n";
                }
            }

            wp_list_categories($args);

            wp_reset_query();
        ?>
    </ul>