title: App crash on resp.getMessage() call
body: <p>i want to get server response during register process, but unfortunately my app keeps crashing here</p>
  
  <pre><code> response.enqueue(new Callback&lt;ServerResponse&gt;() {
          @Override
          public void onResponse(Call&lt;ServerResponse&gt; call, retrofit2.Response&lt;ServerResponse&gt; response) {
  
              ServerResponse resp = response.body();
  //crash occurs here
              Toast.makeText(getApplicationContext(), resp.getMessage(), Toast.LENGTH_LONG).show();
  </code></pre>
  
  <p>and this is what logcat says</p>
  
  <pre><code>java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.chat.ServerResponse.getMessage()' on a null object reference
                                                                at com.chat.RegisterActivity$3.onResponse(RegisterActivity.java:130)
                                                                at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:68)
                                                                at android.os.Handler.handleCallback(Handler.java:739)
                                                                at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                at android.os.Looper.loop(Looper.java:158)
  </code></pre>
  
  <p>below is my serverResponse class</p>
  
  <pre><code>public class ServerResponse {
  
  private String result;
  private String message;
  private User user;
  
  public String getResult() {
      return result;
  }
  
  public String getMessage() {
      return message;
  }
  
  public User getUser() {
      return user;
  }
  }
  </code></pre>