Set Featured Image from first ACF gallery image

//get first image from ACF gallery and set as featured image
// run bulk edit in admin for all posts
add_action( 'save_post', 'my_frontend_save_post');
function my_frontend_save_post( $post_id ) {
    
  $gallery = get_field('images', $post_id, false);
  var_dump($gallery);
  var_dump($post_id);

  if (!empty($gallery)) {
    $image_id = $gallery[0];
    set_post_thumbnail($post_id, $image_id);
  }
}