title: GraphQL mutation returning null on update
body: <p>I am using Amazon's Amplify library for GraphQL. Create mutations are returning
  non-null data in the response, but update mutations return null data in the
  response.</p>
  <pre class="lang-java prettyprint-override"><code>Amplify.API.query(ModelQuery.list(Login.class), response -&gt; {
      boolean isThere = false;
      for (Login login : response.getData()) {
          if (login.getLoginEmail().equals(loginEmail)) {
              isThere = true;
              break;
          }
      }
      if (isThere) {
          Log.e(&quot;MyAmplifyApp&quot;, &quot;Update Query&quot;);
          Amplify.API.mutate(ModelMutation.update(todo),
              response3 -&gt; Log.i(&quot;MyAmplifyApp&quot;, &quot;Updated Todo with id: &quot; + response3.getData().getId()),
              error -&gt; Log.e(&quot;MyAmplifyApp&quot;, &quot;Update failed&quot;, error)
          );
      } else {
          Log.e(&quot;MyAmplifyApp&quot;, &quot;Insert Query&quot;);
          Amplify.API.mutate(ModelMutation.create(todo),
              response2 -&gt; Log.i(&quot;MyAmplifyApp&quot;, &quot;Added Todo with id: &quot; + response2.getData().getId()),
              error -&gt; Log.e(&quot;MyAmplifyApp&quot;, &quot;Create failed&quot;, error)
          );
      }
  }, error -&gt; Log.e(&quot;MyAmplifyApp&quot;, &quot;Query failure&quot;, error));
  </code></pre>
  <p>Error Log:</p>
  <pre class="lang-java prettyprint-override"><code>java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.amplifyframework.datastore.generated.model.Login.getId()' on a null object reference
          at com.example.todo.Activity.LoginActivity.lambda$null$0(LoginActivity.java:722)
          at com.example.todo.Activity.-$$Lambda$LoginActivity$gU4azCKLr7DOG8SII3C8XdBDaxk.accept(lambda)
          at com.amplifyframework.api.aws.AppSyncGraphQLOperation$OkHttpCallback.onResponse(AppSyncGraphQLOperation.java:140)
          at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519)
          at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
          at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
          at java.lang.Thread.run(Thread.java:760)
  </code></pre>