[ZEPPELIN-5436] NPE in ZeppelinClient

### What is this PR for?

Minor PR to fix NPE in ZeppelinClient

### What type of PR is it?
[Improvement ]

### Todos
* [ ] - Task

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-5436

### How should this be tested?
* CI pass

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Jeff Zhang <[email protected]>

Closes #4167 from zjffdu/ZEPPELIN-5436 and squashes the following commits:

100e66a [Jeff Zhang] [ZEPPELIN-5436] NPE in ZeppelinClient

(cherry picked from commit 593a5cc)
Signed-off-by: Jeff Zhang <[email protected]>

Author: zjffdu

zeppelin-client/src/main/java/org/apache/zeppelin/client/ZeppelinClient.java

@@ -88,7 +88,9 @@ private void checkResponse(HttpResponse<JsonNode> response) throws Exception {
     }
     if (response.getStatus() != 200) {
       String message = response.getStatusText();
-      if (response.getBody().getObject().has("message")) {
+      if (response.getBody() != null &&
+              response.getBody().getObject() != null &&
+              response.getBody().getObject().has("message")) {
         message = response.getBody().getObject().getString("message");
       }
       throw new Exception(String.format("Unable to call rest api, status: %s, statusText: %s, message: %s",