Handle not available preferred driver

src/controller/OrderController.php

@@ -70,8 +70,12 @@ class OrderController {
                 return;
             }
             $preferred_driver = $stmt->fetchObject();
+            if ($preferred_driver == false) {
+                $preferred_driver = null;
+            } else {
+                $preferred_driver->id = simpleCrypt($preferred_driver->id, 'e');
+            }
         }
-        $preferred_driver->id = simpleCrypt($preferred_driver->id, 'e');
 
         echo json_encode(array('preferred'=>$preferred_driver , 'result' => $results));