Subsite preview links are broken if the subsite overview is unpublished (causing WSOD)

[andybroomfield]

When a comms officer was trying to share a preview link to a subsite page that wasn't public yet, they encountered a problem for those clicking the link, as they where getting the WSOD technicalk difficulties message.

On closer inspection, the following PHP error was being triggered.

UnexpectedValueException: Object not found in SplObjectStorage->offsetGet() (line 59 of /mnt/www/html/brightonhovemain/docroot/modules/contrib/localgov_subsites/src/Plugin/Block/SubsitesNavigationBlock.php).

This occurs as the subsitenavigationblock is building all menu links using $entities = $mapper->loadAndAccessCheckEntitysForTreeNodes('node', $tree, $cache); and then tries to get the subsite id using $subsite_id = $entities[$ancestors[0]]->id();. But since the overview is unpublished, this will fail as its blank. Wrapping an empty check around this and setting subsite_id ?? NULL if blank does seem to resolve this issue, though it's not an ideal fix.

Steps to replicate

1. Create a subsite overview, but don't publish
2. Create a subsite page that uses the unpublished overview as the parent
3. Generate a preview line for the subsite page
4. Try to use the preview link in a private browsing window.

Expected result Subsite page is shown, albeit with only the page in the navigation menu

Actual resule WSOD, PHP error or technical difficulities message depending on setup.

[andybroomfield]

Additional, wrapping this in an empty block will also cause this error on the twig

The website encountered an unexpected error. Please try again later. AssertionError: $url must be a string or object of type \Drupal\Core\Url in assert() (line 246 of core/lib/Drupal/Core/Template/TwigExtension.php). Becuase the subsite overview link will have no url. so subsite-navigation.html.twig needs to be updated with

{% if item.url is not empty %}
{{ link(item.title, item.url) }}
{% endif %}

[andybroomfield]

Would there be an objection to me applying the first fix?

Wrapping an empty check around this and setting subsite_id ?? NULL if blank does seem to resolve this issue,