Fixes a bug reported in https://core.trac.wordpress.org/ticket/40090

class-walker-nav-menu-40090.php

<?php
/**
 * This is a temporary fix needed to repair the issue mentioned here:
 * https://core.trac.wordpress.org/ticket/40090
 */
add_filter( 'wp_nav_menu_args', 'replace_default_menu_walker', 10, 1 );
function replace_default_menu_walker( $args ) {
  if ( empty($args['walker']) )
    $args['walker'] = new Walker_Nav_Menu_40090;
  return $args;
}

class Walker_Nav_Menu_40090 extends Walker_Nav_Menu {
  public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {
    if ( ! $element )
        return;

    $id_field = $this->db_fields['id'];

    if ( isset( $args[0] ) && is_object( $args[0] ) ) {
      $args[0]->has_children = ! empty( $children_elements[ $element->$id_field ] );
    }

    parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
  }
}