Error when clicking Flutter's DropDwonButton for the second time

My problem is:

1. I fill a DropDownList with records obtained from an API
2. Then I select one of the items and this event fills a second DropDownList, which, like the first one, receives data from an API
3. When I click on the first DropDownList again I get the error:

Code of the first DropDownList

 Widget _regionesContainer() {
    return new Container(
      child: new DropdownButton<String>(
        items: dataRegion.map((item) {
          return new DropdownMenuItem<String>(
            value: item['ID_REGION'].toString(),
            child: new Text(item['NOMBRE_REGION'],
            style: new TextStyle(fontSize: 13.0)
          ),
          );
        }).toList(),
        hint: Text("Seleccione región"),
        onChanged: (String newValReg) {
          setState(() {
            _mySelectionReg = newValReg;
            capturaComuna(newValReg);
            Fluttertoast.showToast(msg: _mySelectionReg,
              fontSize: 18.0,
            );
          });
        },
        value: _mySelectionReg,
      ),
      margin: EdgeInsets.only(bottom: 40.0)
    );
  }

This is the code for the second DropDownList

    Widget _comunasContainer() {
    return new Container(
        child: new DropdownButton<String>(
          items: dataComuna.map((item) {
            return new DropdownMenuItem<String>(
              value: item['ID_COMUNA'].toString(),
              child: new Text(item['NOMBRE_COMUNA'],
                  style: new TextStyle(fontSize: 13.0)),
            );
          }).toList(),
          hint: Text("Seleccione comuna"),
          onChanged: (newValCom) {
            _mySelectionCom = newValCom;
            setState(() {
            });
          },
          value: _mySelectionCom,
        ),
        margin: EdgeInsets.only(bottom: 45.0)
    );
  }

This is the code for the future methods:

String _mySelectionReg;
  String _mySelectionCom;

  String urlRegion = "http://www.aquimaestros.cl/am/Region/Read.php";

  List dataRegion = List(); //Lista de regiones
  List dataComuna = List(); //Lista de comunas

  Future<String> getSWData() async {
    var res = await http
        .get(Uri.encodeFull(urlRegion), headers: {"Accept": "application/json"});
    var resBody = json.decode(res.body);
    setState(() {
      dataRegion = resBody;
    });
    print(resBody);
    return "Sucess";
  }

  Future<String> capturaComuna(String _id) async {
    String urlComuna = "http://www.aquimaestros.cl/am/Comuna/ReadxRegion.php?id_region=${_id}";
    var res = await http
        .get(Uri.encodeFull(urlComuna), headers: {"Accept": "application/json"});
    var resBody = json.decode(res.body);
    setState(() {
      dataComuna = resBody;
    });
    print(resBody);
    return "Sucess";
  }

The error occurs when clicking a second time on the DropDownList of the _regionesContainer widget, this is the error: