WordPress: Require Featured Image

WordPress: Require Featured Image

// from: http://www.codeinwp.com/blog/wordpress-image-code-snippets/
// ============================================================================
// WordPress: Require Featured Image
//
// If you add the following code to your functions.php and try to publish a post that doesn’t have a featured image, 
// you will receive the following message: “You must select a Featured Image before your Post can be published.”
// ============================================================================

add_action('save_post', 'wpds_check_thumbnail');
add_action('admin_notices', 'wpds_thumbnail_error');

function wpds_check_thumbnail($post_id) {

    // change to any custom post type 
    if(get_post_type($post_id) != 'post')
        return;
    
    if ( !has_post_thumbnail( $post_id ) ) {
        // set a transient to show the users an admin message
        set_transient( "has_post_thumbnail", "no" );
        // unhook this function so it doesn't loop infinitely
        remove_action('save_post', 'wpds_check_thumbnail');
        // update the post set it to draft
        wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));

        add_action('save_post', 'wpds_check_thumbnail');
    } 
    else {
        delete_transient( "has_post_thumbnail" );
    }
}

function wpds_thumbnail_error() {
    // check if the transient is set, and display the error message
    if ( get_transient( "has_post_thumbnail" ) == "no" ) {
        echo "<div id='message' class='error'><p><strong>You must select Featured Image. Your Post is saved but it can not be published.</strong></p></div>";
        delete_transient( "has_post_thumbnail" );
    }
}

This does not seem to work with Gutenberg