I also wanted to test the new experimental sidebar navigation in one of my projects and I got an error and The website was unreachable. But in my case the error was different:
"PHP message: Uncaught PHP Exception Drupal\Component\Plugin\Exception\PluginNotFoundException: "The "media_type" entity type does not exist." at /home/workshop/drupal/d10/test/web/core/lib/Drupal/Core/Entity/EntityTypeManager.php line 139"
At first, I tried it on a Drupal 9 site and I got the above error, then I tried it on a fresh Drupal 10 installation and I got the same error and I site was not reachable.
💬 | Simple XML sitemap | Error: Class "Drupal\simple_sitemap\Controller\SimplesitemapController" not found in include() (line 17 of /var/www/html/web/modules/contrib/domain_simple_sitemap/src/Controller/DomainSimpleSitemapController.php)
I am also facing the same issue on my Drupal 9 site.