Error " The body might complete normally, causing 'null' to be returned, but the return type, " in Flutter

In my code when open this page then call fetchData() method in initState.fetchData methos call every 15 seconds.
initState

Timer? timer;
  Future<List<DataDoctor>>? future;
  @override
  void initState() {
    super.initState();

    timer = Timer.periodic(Duration(seconds: 15), (Timer t) => fetchData());
  }

in fetchData method geting data from API call in the API has
these data.

[![enter image description here][2]][2] those data should retrieve to backend for now I only retrieve only Boolean value.
like as my code this Boolean value I get as doctor_in variable. if doctor_in false then should recall again API and should get data. it should at every 15 seconds until open this page.

data retrieve code

  FutureBuilder<DataDoctor> fetchData() {
    builder:
    (context, snapshot) {
      if (snapshot.hasData) {
        if (snapshot.data!.doctor_in == false) {
          future = client.isDoctorActive();
        }
      } else if (snapshot.hasError) {
        return Text('${snapshot.error}');
      }
    };
  }

model

class DataDoctor {
  String channelName;
  String receiver_name;
  bool doctor_in;

  DataDoctor(
      {required this.channelName,
      required this.receiver_name,
      required this.doctor_in});

  factory DataDoctor.fromJson(Map<String, dynamic> json) {
    return DataDoctor(
      channelName: json['Mobile'] == null ? null : json['Mobile'],
      receiver_name: json['Name'] == null ? null : json['Name'],
      doctor_in: json['Boolean'] == null ? null : json['Boolean'],
    );
  }
}

api call

class ApiServiceDataDoctor {
  final String url =
      'https://tools.learningcontainer.com/sample-json-file.json';
  Future<List<DataDoctor>> isDoctorActive() async {
    Response response = await get(Uri.parse(url));
    if (response.statusCode == 200) {
      Map<String, dynamic> json = jsonDecode(response.body);

      Map<String, dynamic> body = json['dataActive'];
      List<DataDoctor> datas = [DataDoctor.fromJson(body)];
      return datas;
    } else {
      throw ('cannot fetch data');
    }
  }
}

if doctor_in == true then ElevatedButton should enable