Error message in checkout step I'm getting the following error on the return page of my off-site payment:

#1 ... /httpdocs/core/modules/editor/editor.module(554): _drupal_error_handler(4096, 'Argument 1 pass...', '... /httpdocs/ ...', 554, Array)
#2 ... /httpdocs/core/modules/editor/editor.module(384): _editor_get_file_uuids_by_field(NULL)
#3 [internal function]: editor_entity_update(Object(Drupal\commerce_order\Entity\Order))
#4 ... /httpdocs/core/lib/Drupal/Core/Extension/ModuleHandler.php(402): call_user_func_array('editor_entity_u...', Array)
#5 ... /httpdocs/core/lib/Drupal/Core/Entity/EntityStorageBase.php(169): Drupal\Core\Extension\ModuleHandler-&gt;invokeAll('entity_update', Array)
#6 ... /httpdocs/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php(730): Drupal\Core\Entity\EntityStorageBase-&gt;invokeHook('update', Object(Drupal\commerce_order\Entity\Order))
#7 ... /httpdocs/modules/contrib/commerce/src/CommerceContentEntityStorage.php(88): Drupal\Core\Entity\ContentEntityStorageBase-&gt;invokeHook('update', Object(Drupal\commerce_order\Entity\Order))
#8 ... /httpdocs/core/lib/Drupal/Core/Entity/EntityStorageBase.php(470): Drupal\commerce\CommerceContentEntityStorage-&gt;invokeHook('update', Object(Drupal\commerce_order\Entity\Order))
#9 ... /httpdocs/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php(615): Drupal\Core\Entity\EntityStorageBase-&gt;doPostSave(Object(Drupal\commerce_order\Entity\Order), true)
#10 ... /httpdocs/core/lib/Drupal/Core/Entity/EntityStorageBase.php(395): Drupal\Core\Entity\ContentEntityStorageBase-&gt;doPostSave(Object(Drupal\commerce_order\Entity\Order), true)
#11 ... /httpdocs/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php(820): Drupal\Core\Entity\EntityStorageBase-&gt;save(Object(Drupal\commerce_order\Entity\Order))
#12 ... /httpdocs/core/lib/Drupal/Core/Entity/Entity.php(391): Drupal\Core\Entity\Sql\SqlContentEntityStorage-&gt;save(Object(Drupal\commerce_order\Entity\Order))
#13 ... /httpdocs/modules/contrib/commerce/modules/checkout/src/Plugin/Commerce\CheckoutFlow\CheckoutFlowBase.php(130): Drupal\Core\Entity\Entity-&gt;save()
#14 ... /httpdocs/modules/contrib/commerce/modules/payment/src/Controller/PaymentCheckoutController.php(88): Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowBase-&gt;redirectToStep('complete')
#15 [internal function]: Drupal\commerce_payment\Controller\PaymentCheckoutController-&gt;returnPage(Object(Drupal\commerce_order\Entity\Order), Object(Symfony\Component\HttpFoundation\Request))


(any stack beyond this is, afaik, Drupal boiler plate)

Digging into editor.module, obviously the original property of my order entity is NULL, and that causes the error. 

Does this indicate that my order has not been updated, and does not need saving? As in, I should just patch the CheckoutFlowBase class to check if the original property is set, and if so, save?

Or is the fix going to have to be more complicated?
That error most commonly occurs if an order is being double-saved, by calling $order->save() within a hook_entity_presave() or a matching event, including pre/post transition events.

If you need to modify the order during a state transition, do it in a pre-transition event and let the system take care of the rest.