Display post navigation only for posts with a particular tag

Hi everyone! on my new website I use posts tagged with the "portfolio" term to create a simple portfolio.

I want display the post navigation only in these tagged posts, I use the solutions found in the forum, and it works fine,

This is my code:

// Add footer post navigation only for portfolio post.
add_filter( 'generate_post_navigation_args', function( $args ) {

  if ( is_single() && has_tag('portfolio')) {
    $prevPost = get_previous_post(true);
    $prevThumbnail = isset($prevPost) ? get_the_post_thumbnail($prevPost->ID) : '';
    $nextPost = get_next_post(true);
    $nextThumbnail = isset($nextPost) ? get_the_post_thumbnail($nextPost->ID) : '';

    $args['previous_format'] = '<div class="nav-previous"><a href="' . get_permalink($prevPost->ID) . '" rel="prev">' . $prevThumbnail . '<img class="arrow" alt="prev" src="' . get_stylesheet_directory_uri() . '/assets/media/icons/chevron-icon.svg"><span class="prev" title="' . esc_attr__('Previous', 'generatepress') . '">' . get_the_title($prevPost->ID) . '</a></span></div>'; 
    $args['next_format'] = '<div class="nav-next"><a href="' . get_permalink($nextPost->ID) . '" rel="next"><span class="next" title="' . esc_attr__('Next', 'generatepress') . '">' . get_the_title($nextPost->ID) . '</span><img class="arrow" alt="next" src="' . get_stylesheet_directory_uri() . '/assets/media/icons/chevron-icon.svg">' . $nextThumbnail . '</a></div>';
  }

  return $args;
} );

and:

// Add footer code with customized post navigation.
add_action('generate_after_content', 'custom_footer_meta_portfolio');
function custom_footer_meta_portfolio() {
  if (is_single() && has_tag('portfolio')) : ?>
    <footer class="entry-meta">
      <?php generate_content_nav('nav-below'); ?>
    </footer><!-- .entry-meta -->
  <?php endif;
}

But in the first and last post tagged with "portfolio" it doesn't work, and appears this error:

Notice: Trying to get property 'ID' of non-object in/Users/campoy.pro/macbook-pro-de-jose.local:5757/wp-content/themes/dev/functions.php on line 467.

Could you help me with my code?

Thanks in advance!