Print Post Parent Title

You can just add a check:

if ( ! empty ( $post->post_parent ) )
{
    $parent_title = get_the_title($post->post_parent);
    echo "<h1>$parent_title</h1>";
}

or use setup_postdata():

setup_postdata( $post->post_parent );
the_title( /* arguments here*/ );
wp_reset_postdata();