Hi Pavel,

The menu walker presented in the tutorial is designed to work with the navbar. If you want to create a SideNav menu walker you have to modify the walker from the tutorial a little bit because the SideNav is made of different HTML tags. Example implementation could look like this:

class Sidenav_Menu_Walker extends Walker {

  public $tree_type = array( 'post_type', 'taxonomy', 'custom' );

  public $db_fields = array( 'parent' => 'menu_item_parent', 'id' => 'db_id' );

  public function start_lvl( &$output, $depth = 0, $args = array() ) {
    $indent = str_repeat("\t", $depth);
    $output .= "\n$indent <div class=\"collapsible-body\"> <ul class=\"sub-menu\">n";
  }

  public function end_lvl( &$output, $depth = 0, $args = array() ) {
    $indent = str_repeat("\t", $depth);

    $output .= "$indent</ul></div>\n";
  }

  function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
    $id_field = $this->db_fields['id'];
    if ( is_object( $args[0] ) ) {
      $args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
    }
    return parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
  }

  public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {

    $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

    $classes = empty( $item->classes ) ? array() : (array) $item->classes;
    $classes[] = ' menu-item-' . $item->ID;

    $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
    $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

    $id      = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args, $depth );
    $link_id = $id ? ' id="link-' . esc_attr( $id ) . '"' : '';
    $id      = $id ? ' id="' . esc_attr( $id ) . '"' : '';

    $output .= $indent . '<li' . $id . $class_names .'>';

    $atts = array();
    $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
    $atts['target'] = ! empty( $item->target )     ? $item->target     : '';
    $atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
    $atts['href']   = ! empty( $item->url )        ? $item->url        : '';

    $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args, $depth );

    $attributes = '';
    foreach ( $atts as $attr => $value ) {
      if ( ! empty( $value ) ) {
        $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
        $attributes .= ' ' . $attr . '="' . $value . '"';
      }
    }

    $active = ($item->current || $item->current_item_ancestor) ? 'active' : '';

    $item_output = $args->before;
    if ($args->has_children ) {
      $item_output .= '<a class="collapsible-header waves-effect arrow-r '.$active.'">';
    }else {
      $item_output .= '<a class="collapsible-header waves-effect"' . $link_id . ' '. $attributes .'>';
    }
    $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
    if ($args->has_children ) {
      $item_output .= '<i class="fa fa-angle-down rotate-icon"></i></a>';
    }else {
      $item_output .= '</a>';
    }

    $item_output .= $args->after;

    $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
  }

  public function end_el( &$output, $item, $depth = 0, $args = array() ) {

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

  }

}

And to use it, you have to write something like this:

$args = array(
    'theme_location'  => 'primary',
    'container'       => 'false',
    'echo'            => true,
    'menu_class'      => 'collapsible collapsible-accordion',
    'menu_id'         => 'side-menu',
    'fallback_cb'     => 'wp_page_menu',
    'walker'          => new Sidenav_Menu_Walker()
);
wp_nav_menu( $args );

When you refresh the page you should see your awesome sidenav.

Regards,
Sebastian