If you edit an existing Landing Page, or create a new one, and empty the "Subcategory links" URL field, attempts to view that node will display the error message:
The website encountered an unexpected error. Please try again later.
This is the field in the Admin UI:
Watchdog reports a PHP error when trying to view the node.
InvalidArgumentException: The URI '' is invalid. You must use a valid URI scheme. in Drupal\Core\Url::fromUri() (line 293 of /code/docroot/core/lib/Drupal/Core/Url.php).
If you edit an existing Landing Page, or create a new one, and empty the "Subcategory links" URL field, attempts to view that node will display the error message:
This is the field in the Admin UI:
Watchdog reports a PHP error when trying to view the node.