Error response - Cannot display response in Dialogflow simulator.

[alvinak]

Please help to solve this issue:
I am getting following response for a DialogFlow Rich message

Cannot display response in Dialogflow simulator. Please test on the Google Assistant simulator instead.

Below is my code in flutter

df.AuthGoogle authGoogle = await df.AuthGoogle(fileJson: 'asset/my_json.json').build();
    df.Dialogflow dialogflow = df.Dialogflow(authGoogle: authGoogle, sessionId: '123456');
    df.DetectIntentResponse response = await dialogflow.detectIntentFromText('Hi', Language.english,);
for (df.Message message in response.queryResult.fulfillmentMessages) {
      print(message.toJson());
    }

In Dialogflow, response is added in the Fulfillment part
Below is the code written inside Dialogflow fullfillment -

const app = dialogflow({debug: true});
// Handle the Dialogflow intent named 'welcomeMessage'.
app.intent('welcomeMessage', (conv) => {
  
  conv.ask('I can help you.  Would you like to continue? ');
  
});

How to access the response coming from fullfillment ?