DIO response decode issue

Your response data is already a Map so you could simply do that:

var response = await dio.get(_url);
var responseBody = response.data;
print(responseBody);

This is with explicit types:

Response<Map> response = await dio.get(_url);
Map responseBody = response.data;
print(responseBody);

Follow this approach Take the response as String. Now you can decode it...

 Response<String> response = await DioUtils.getInstance().get(JSON_API);
 List responseJson = json.decode(response.data);
 return responseJson.map((m) => new User.fromJson(m)).toList();