Fix NPE

Author: aiwenmo

dinky-gateway/src/main/java/org/dinky/gateway/kubernetes/KubernetesGateway.java

@@ -237,8 +237,10 @@ public TestResult test() {
             return TestResult.success();
         } catch (Exception e) {
             logger.error(Status.GATEWAY_KUBERNETES_TEST_FAILED.getMessage(), e);
+            String errorDetail =
+                    e.getMessage() != null ? e.getMessage() : e.toString();
             return TestResult.fail(
-                    StrFormatter.format("{}:{}", Status.GATEWAY_KUBERNETES_TEST_FAILED.getMessage(), e.getMessage()));
+                    StrFormatter.format("{} {}", Status.GATEWAY_KUBERNETES_TEST_FAILED.getMessage(), errorDetail));
         } finally {
             close();
         }