Custom Nav widget disappears when showing custom post type on archive pages

Hi there,

I’m having some trouble showing the Custom Navigation widget on archive pages.

My twenty twelve child theme has a custom post type that makes use of the regular post type categories. As I want my custom post type posts to appear on category archive pages I had to add the code below to my functions.php:

// add custom post type to category archive pages
function namespace_add_custom_types( $query ) {
  if( is_category() || is_tag() && empty( $query->query_vars['suppress_filters'] ) ) {
    $query->set( 'post_type', array(
     'post', 'deelnemer', 'category'
		));
	  return $query;
	}
}
add_filter( 'pre_get_posts', 'namespace_add_custom_types' );

Now this piece of code is causing my Custom Navigation widget to disappear (at least, the links the widget generates disappear).

Any ideas what I could do to solve this in the cleanest possible way?

Thanks in advance!