Custom Post Types in Tag Page make Manu disapear

Hi Guys,

I use the following snippet in my functions.php to add custom post types to my tag page.

add_filter('pre_get_posts', 'custom_post_types_in_results');

function custom_post_types_in_results($query) {

	if(is_tag()) {

		$post_type = get_query_var('post_type');

		if($post_type)
			$post_type = $post_type;
		else
			$post_type = array('nieuws', 'diensten', 'referenties', 'menu');

		$query->set('post_type',$post_type);

		return $query;
	}

}

However, because of this snippet the menu’s are not being displayed. Which is logic because I edit every query on the page to these specific post types.

I had in mind that I could solve this by adding the menu post type to it, but I cannot seem to find what the post type for menu items is.

How can help me solve this problem? Or I need to be able to minimize the function to the actual search results and not all queries. OR I need to add the menu post type to this change. Who knows?