Undefined variable $default_featured_media just only on one post

Warning: Undefined variable $default_featured_media in?/data/www/25398/365tipu_cz/www/wp-content/plugins/simple-tags/inc/class.client.related_posts.php?on line?347

This is interesting, it shows this error just for one post. This post has everything as other posts – tags (used for related posts), and has featured media. So i a really wonder, why this code triggers:

    if (empty($post_thumbnail_url)) {
        $post_thumbnail_url = $default_featured_media;
    }

I can switch off and it diasppears.

“Default post thumb” is not actived in settings. A

Maybe you will be able to find the bug. If any help needed, i am here to answer any questions

Daniel

The page I need help with: [log in to see the link]