obache

change_url_to_required() of class-googlesitemapgeneratorloader.php:

					else if($postType[0] === 'post_tag' || $postType[0] === 'category' ) return 'params=tax-' . $postType[0] . '-' . $matches[0];

Replace $postType[0] === 'post_tag' || $postType[0] === 'category' with taxonomy_exists($postType[0]), and custom taxonomy sitemap file will be not empty.

It is ad-hoc fix, because custom taxonomies may be disabled for sitemap, or custom post type with same slug may exist.

Above suggested changes are wrong.

Just change do_robots() from $sm_url = $this->get_xml_url(); to $sm_url = $this->get_base_sitemap_url();.