LogicException in unsaved subqueues when adding multiple items.

Steps to reproduce:

# Add a new queue of the type “Multiple subqueues”.
# Add a new subqueue.
# Add a new item without saving the queue.
# Add a second item without saving the queue.
# When clicking “Add another item” is 3rd item field not added.
# Saving the subqueue will save the first item added, but the second item is lost.

After saving the subqueue is it working fine again. So it looks like it happens only when the subqueue was added and not saved.

When the issue occurs is the following error logged in the watchdog:

LogicException: Settings can not be serialized. This probably means you are serializing an object that has an indirect reference to the Settings object. Adjust your code so that is not necessary. in Drupal\Core\Site\Settings->__sleep() (line 62 of /core/lib/Drupal/Core/Site/Settings.php).

Thanks for the detailed steps to reproduce, it's now fixed with the patch attached.