Update Xero.php

It is possible for response to be null so ensure we check before calling

src/Xero.php

@@ -320,7 +320,7 @@ protected function guzzle(string $type, string $request, array $data = [], bool
             ];
         } catch (RequestException $e) {
             $response = json_decode($e->response->body());
-            throw new Exception($response->Detail ?? "Type: $response->Type Message: $response->Message Error Number: $response->ErrorNumber");
+            throw new Exception($response->Detail ?? "Type: $response?->Type Message: $response?->Message Error Number: $response?->ErrorNumber");
         } catch (Exception $e) {
             throw new Exception($e->getMessage());
         }