Link format gotten from API response

I am testing to call RASA API by using Flutter. When I get the response with link inside (Like the default image when we call mood_unhappy intent), the link is displayed with the backslash before the original slash like this:

[
{"recipient_id":"user","text":"Here is something to cheer you up:"},
{"recipient_id":"user","image":"https:\/\/i.imgur.com\/nGF1K8f.jpg"},
{"recipient_id":"user","text":"Did that help you?"}
]

I also test the API on POSTMAN and it still working fine

[
    {
        "recipient_id": "test_user",
        "text": "Here is something to cheer you up:"
    },
    {
        "recipient_id": "test_user",
        "image": "https://i.imgur.com/nGF1K8f.jpg"
    },
    {
        "recipient_id": "test_user",
        "text": "Did that help you?"
    }
]

This is my Flutter code to call the API

void postMessage(message) async {
    final uri = Uri.parse("http://localhost:5005/webhooks/rest/webhook");
    final response = await http.post(
      uri,
      headers: <String, String>{
      'Content-Type': 'application/json; charset=UTF-8',
    },
      body: jsonEncode(message),
    );
    debugPrint(response.body);
  }

Is there any way to make the link displayed as usual like in Postman? Is that the Flutter’s Problem or Rasa’s problem? I am taking doubt about Flutter’s problem but I cannot find any solution about this.

Edit: I have tested another API on Flutter and the link still appear in usual way. So, the problem seem came from Rasa.

Is URL causing any problem? I tried it on browser and it is working fine. These slashes \ are escape characters and they won’t make anything different in your code.