next/pre navigation problem in single-gallery.php

Hi Brad,

thanks for your answer!
Anyway sadly I tried this before.
And what is so strange about it is that I can type in there any number I want, at once the next/previous titles (nav) disappear completely. For me there is no logic in this behavior.
For when the ID typed there would not exist nothing should be excluded = everything should be as it was.
Maybe it has to do something with the rest of the code here:

<?php
                                $prevPost = get_previous_post(false, XXX);
                                $nextPost = get_next_post(false, XXX);

                                $root_first_categ = get_post_categories($post->ID, $only_first_cat = true, $taxonomy = 'gallery-category', $margin_elem_start = '', $margin_elem_end = ' ', $delimiter = ', ', $a_class = 'icon-root', $show_cat_name = false);
                            ?>
                            <div class="post-navigation">

<ul>
<li>
                                        <?php
                                            if ($prevPost != '') {
                                                previous_post_link('<i class="icon-prev"></i> %link');
                                            }
                                        ?>
                                    </li>
                                    <?php
                                        if(strlen($root_first_categ)){
                                            echo '
<li>'.$root_first_categ.'</li>
';
                                        }
                                    ?>

<li>
                                        <?php
                                            if ($nextPost != '') {
                                                next_post_link('%link <i class="icon-next"></i>');
                                            }
                                        ?>
                                    </li>
</ul>
                            </div>
                            <div class="clear bottom-separator"></div>
                            <?php wp_link_pages(array('before' => '<div class="pagenumbers"><p>Pages:','after' => '</p></div>  ', 'next_or_number' => 'number'));  ?>
                            <?php get_template_part('social-sharing'); ?>
                        </div>

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