New answers tagged nonce
Whoever wrote your theme didn't bother to verify the existence of array keys before using them. The error is happening because the key album_tracks_metabox_nonce doesn't exist in the $_POST array. The line likely should be: if ( !isset($_POST['album_tracks_metabox_nonce']) || !wp_verify_nonce($_POST['album_tracks_metabox_nonce'], ...
Top 50 recent answers are included