fix(NPE): Corrected NPE error.

see #JENKINS-41436

Author: hypery2k

Diff for: src/main/java/jenkins/plugins/rocketchatnotifier/rocket/RocketChatClientImpl.java

@@ -91,7 +91,7 @@ public void send(String channelName, String message) throws ValidatorException,
     Response res = this.callBuilder.buildCall(RocketChatRestApiV1.PostMessage,
       null, body);
     if (!res.isSuccessful()) {
-      LOG.severe("Could not sebd message: " + res.getMessage().getMsg());
+      LOG.severe("Could not send message: " + res.getMessage() != null ? res.getMessage().getMsg() : "");
       throw new IOException("The send of the message was unsuccessful.");
     }
   }