[WordPress] How to programmatically set a default menu in WordPress.

fallback-cb.php

<?php
/**
 * Provides a default menu featuring the 'Home' link, if not other menu has been provided.
 *
 * @package     Acme
 * @subpackage  inc
 * @version     1.0.0
 * @since       1.0.0
 */
function acme_default_menu() {

	$html = '<ul id="acme-default-menu">';
		$html .= '<li class="menu-item menu-item-type-post_type menu-item-object-page">';
			$html .= '<a href="' . esc_url( home_url() ) . '" title="' . __( 'Home', 'acme' ) . '">';
				$html .= __( 'Home', 'acme' );
			$html .= '</a>';
		$html .= '</li>';
	$html .= '</ul>';

	echo $html;

} // end acme_default_menu

final-wp-nav.php

<?php
wp_nav_menu(
	array(
		'theme_location' => 'main',
		'fallback_cb'    => 'acme_default_menu'
	)
);

initial-wp-nav-menu.php

<?php
wp_nav_menu(
	array(
		'theme_location' => 'main'
	)
);