ENGCOM-4361: [Backport] Issue #20380 fixed for 2.2 #20866

Author: sidolov

app/code/Magento/Sales/Model/Order.php

@@ -1298,12 +1298,12 @@ public function getTrackingNumbers()
      * Retrieve shipping method
      *
      * @param bool $asObject return carrier code and shipping method data as object
-     * @return string|\Magento\Framework\DataObject
+     * @return string|null|\Magento\Framework\DataObject
      */
     public function getShippingMethod($asObject = false)
     {
         $shippingMethod = parent::getShippingMethod();
-        if (!$asObject) {
+        if (!$asObject || !$shippingMethod) {
             return $shippingMethod;
         } else {
             list($carrierCode, $method) = explode('_', $shippingMethod, 2);