Flutter dropdown button form field error - with two dropdown

I am new to programming with flutter, and a problem has arisen that, after reading a lot in this forum, I have not managed to solve.

 I have two classes, one Provinces and another cities, each of them are obtained through an API query.

First I get the provinces with a webservices. and I mount with a futurebuilder the dropdown with the provinces.

Once the province is selected, and through another web service to which we pass the province, we obtain the cities through a different web service.

and with a FutureBuilder I create the dropdown with the cities of the requested province.

Everything seems correct .... but once I have the list of cities, if we select a province other than the one selected, I get the following error:

=============================================================================================

There should be exactly one item with [DropdownButton]'s value: Instance of 'Municipio'.

Either zero or 2 or more [DropdownMenuItem]s were detected with the same value 'package:flutter/src/material/dropdown.dart':

Failed assertion: line 803 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) {

return item.value == value; }).length == 1'

=============================================================================================

@override
void initState() {
_provincias = provinciasProvider.getProvincias();

super.initState();
}



Widget _dropDownProvincias() {
return FutureBuilder(
    future: _provincias,
    builder: (context, snapshot) {
      if (snapshot.hasError) return Text(snapshot.error);

      if (snapshot.hasData) {
        return DropdownButtonFormField(
          isExpanded: false,
          isDense: true,
          decoration: new InputDecoration(
            focusedBorder: OutlineInputBorder(
              borderSide: BorderSide(color: Colors.greenAccent, width: 5.0),
            ),
            enabledBorder: OutlineInputBorder(
              borderSide: BorderSide(color: Colors.teal, width: 1.0),
            ),
            labelText: 'Seleccione Provincia',
            labelStyle: TextStyle(color: Colors.black),
            //icon: Icon(Icons.language)
          ), //, color: Colors.white10
          items: snapshot.data
              .map<DropdownMenuItem<Provincia>>((Provincia provincia) {
            return DropdownMenuItem<Provincia>(
              value: provincia,
              child: Text(provincia.nombreProvincia,
                  style: TextStyle(color: Color.fromRGBO(58, 66, 46, .9))),
            );
          }).toList(),

          onChanged: (Provincia newValue) {
            setState(() {
              selectedProvincia = newValue;
              codigoProvinciaSeleccionada = newValue.codigoProvincia;
              nombreProvinciaSeleccionada = newValue.nombreProvincia;
              _municipios = municipiosProvider
                  .getMunicipios(codigoProvinciaSeleccionada);
            });

            print(codigoProvinciaSeleccionada);
            print(newValue.nombreProvincia);
          },
        );
      }
      return Container();
    });




Widget _dropDownPoblacion(String codigoProvinciaSeleccionada) {
return FutureBuilder(
    future: _municipios,
    builder: (context, snapshot) {
      if (snapshot.hasError) return Text(snapshot.error);

      if (snapshot.hasData) {
        return DropdownButtonFormField(
          isExpanded: true,
          isDense: true,
          decoration: new InputDecoration(
            focusedBorder: OutlineInputBorder(
              borderSide: BorderSide(color: Colors.greenAccent, width: 5.0),
            ),
            enabledBorder: OutlineInputBorder(
              borderSide: BorderSide(color: Colors.teal, width: 1.0),
            ),
            labelText: 'Seleccione Localidad',
            labelStyle: TextStyle(color: Colors.black),
            //icon: Icon(Icons.language)
          ), //, color: Colors.white10
          items: snapshot.data
              .map<DropdownMenuItem<Municipio>>((Municipio municipio) {
            return DropdownMenuItem<Municipio>(
              value: municipio,
              child: Text(municipio.nombreMunicipio,
                  style: TextStyle(color: Color.fromRGBO(58, 66, 46, .9))),
            );
          }).toList(),

          onChanged: (Municipio newValue) {
            setState(() => selectedMunicipio = newValue);
            codigomunicipioSeleccionado = newValue.codigoMunicipio;
            nombreMunicipioSeleccionado = newValue.nombreMunicipio;
            print(codigomunicipioSeleccionado);
            print(newValue.nombreMunicipio);
          },
        );
      }
      return Container();
    });

Thank for us help¡¡¡

The solution for me was to change DropdownButtonFormField to DropdownButton.

From there everything worked.