In my setup with commerce 2.0 using a birthday field in the user profile payment method where the value is NULL by intention, it throws the following error (only the first time - recalling with ajax - thus changing the payment method brings it up, the value is somehow set).
Non ajax will lead to a permanent error (maybe reloading the initialized form helps ..)
Argument 1 passed to Drupal\Component\Datetime\DateTimePlus::createFromArray() must be of the type array, null given, called in /.../web/core/lib/Drupal/Core/Datetime/Element/Datelist.php on line 65
This can be fixed with a simple NULL check.