A quick explanation of how to use the WordPress wp_nav_menu's fallback_cb variable.

gistfile1.php

<?php 
    wp_nav_menu( array(
        'menu'        => 'top_menu',
        'depth'       => 2,
        'container'   => false,
        'menu_class'  => 'nav',
        //Process nav menu using our custom nav walker
        'walker'      => new twitter_bootstrap_nav_walker()),
        'fallback_cb' => 'wp_page_menu', // No menu available: will display all pages in menu
        //'fallback_cb'    => '__return_false', // No menu available: no output.
    );
?>

In order to prevent an error when no menu is defined you must declare a fallback_cb variable inside your themes wp_nav_menu array. You have two options for alternate display that come with wordpress.

'fallback_cb' => 'wp_page_menu'

This will populate the menu with your WordPress pages if no menu is assigned in the WordPress admin.

'fallback_cb' => '__return_false'

This will return nothing if no menu is assigned in the WordPress admin.

I think the 'fallback_cb' must be set inside the array, the code above is set outside array, look

new twitter_bootstrap_nav_walker()),

Thanks :)