Error "Trying to get property of non-object" with Custom Walker for wp_nav_menu

Question

I am trying to add some ARIA related stuff to the wp_nav_menu function. I use a custom walker class for this purpose:

class Walker_Nav_Menu_With_Aria extends Walker_Nav_Menu {
        function start_lvl( &$output, $depth = 0, $args = array() ) {
                $indent = str_repeat("\t", $depth);
                $output .= "\n$indent<ul class=\"sub-menu\" role=\"group\">\n";
        }

        function end_lvl( &$output, $depth = 0, $args = array() ) {
                $indent = str_repeat("\t", $depth);
                $output .= "$indent</ul>\n";
        }

        function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
                $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

                $class_names = $value = '';

                $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 ) );
                $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

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

                // Add attributes for <li>
                $li_attributes  = ' role="treeitem"';
                $li_attributes .= ' aria-expanded="false"';

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

                $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
                $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
                $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
                $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

                // Add attributes for <a>
                $attributes .= $depth == 0 ? ' tabindex="0"' : ' tabindex="-1"';

                $item_output = $args->before;
                $item_output .= '<a'. $attributes .'>';
                $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
                $item_output .= '</a>';
                $item_output .= $args->after;

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

        function end_el( &$output, $item, $depth = 0, $args = array() ) {
                $output .= "</li>\n";
        }
}

I just copy-pasted the original Walker_Nav_Menu class and added stuff, but I am getting errors because the $args variable treated like an object is really an array. But the really weird thing is that this error persists, even if I pass in the original walker like this:

wp_nav_menu( array(
    'theme_location' => 'main-nav',
    'walker'         => new Walker_Nav_Menu,
) );

These are the error messages I get:

NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 88
NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 90
NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 90
NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 92

The lines look like this (In the original file!):

(88) $item_output = $args->before;
(89) $item_output .= '<a'. $attributes .'>';
(90) $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
(91) $item_output .= '</a>';
(92) $item_output .= $args->after;

Answer 1

I get this error when there are no menus defined or no menus set for the location at Appearance->Menus. When that occurs wp_nav_menu uses a page walker fallback.

1. The fallback (default) for wp_nav_menu is wp_walker_page
2. which uses wp_page_menu
3. which uses wp_list_pages
4. which uses walk_page_tree
5. which uses Walker_Page not Walker_Nav_Menu.

And apparently the two walkers are not compatible. I don't know why it doesn't fail gracefully. That seems like a bug to me.

With a menu set at wp-admin->Appearance->Menus, your code works.

You can avoid the error by checking to see that there is a menu assigned to the location before trying to use the location.

$locations = get_nav_menu_locations();
if (0 !== $locations['main-nav']) {
  wp_nav_menu( array(
      'theme_location' => 'main-nav',
      'walker'         => new Walker_Nav_Menu_With_Aria,
  ) );
}

Or, if you'd prefer less antihistamine addled code ( thanks @Rarst ):

if (has_nav_menu('primary')) {
  wp_nav_menu( array(
      'theme_location' => 'primary',
      'walker'         => new Walker_Nav_Menu_With_Aria,
  ) );
}

Answer 2

use below code to repair :

add_filter( 'wp_get_nav_menu_object', 'override_wp_get_nav_menu_object', 10, 2 );
function override_wp_get_nav_menu_object( $menu_obj, $menu ) {

    if ( ! is_object( $menu_obj ) ) {
        $menu_obj = (object) array( 'name' => '' );
    }

    return $menu_obj;
}