Issue #3163698 by JeroenT, SenneS: _clone method called on non-object when...

    // Save the modified shipments.

    $shipments = [];

    foreach (Element::children($pane_form['shipments']) as $index) {

      if (!isset($pane_form['shipments'][$index]['#shipment'])) {

        continue;

      }

      /** @var \Drupal\commerce_shipping\Entity\ShipmentInterface $shipment */

      $shipment = clone $pane_form['shipments'][$index]['#shipment'];

      $form_display = EntityFormDisplay::collectRenderDisplay($shipment, 'checkout');