<?php  if(wp_nav_menu( array( 'theme_location' => 'main_menu','fallback_cb' =>false,'echo' =>false))){
    echo '<div id="categories_strip">';
    wp_nav_menu( array( 'theme_location' => 'main_menu','fallback_cb' =>'nav_nav','echo' =>true, 'container' => 'div','container_id'  => 'main_nav_menu'));
    dynamic_sidebar('Header Navigation Right');
    echo '</div>';
}else{
    ?>