我实现了两个下拉列表。当我选择州和地区时，它工作得很好。当我更改状态时，它显示错误。在getDistricts（）方法中设置区域下拉列表值为空。请帮助并提前表示感谢。

在控制台得到此错误：

应该只有一个项目具有[DropdownButton]的值：1.检测到零个或2个或更多个[DropdownMenuItem]具有相同值“package：flutter/src/material/dropple. dart”：Assert失败：第828行位置15：'项目== null|| items.isEmpty ||value == null|| items.where（（DropdownMenuItem item）{ return item.value == value;}）.length == 1'

class AddAddress extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return AddAddressPage();
  }
}
class AddAddressPage extends StatefulWidget {
  @override
  _AddAddressPageState createState() => _AddAddressPageState();
}
class _AddAddressPageState extends State<AddAddressPage> {
  bool loader = false;
  int intState;
  int intDistrict;
  List<Location> districts=listDistricts;
  List<Location> states=listStates;
  getDistricts()async{
    setState(() {
      loader=false;
    });
  List<Location> district= await service.getDistrictsByStateId(intState);
   setState(() {
     districts=district;
      loader=false;
    });
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Address"),
        backgroundColor: Colors.white,
        centerTitle: true,
      ),
      body: Stack(
        children: <Widget>[
          SingleChildScrollView(
            child: Padding(
              padding: const EdgeInsets.only(top: 10.0, left: 30, right: 30),
              child: Form(
                child: Container(
                  child: Column(
                    children: <Widget>[
                      Padding(
                        padding: const EdgeInsets.only(top: 15.0),
                        child: DropdownButtonFormField<int>(
                            decoration: InputDecoration(
                              contentPadding:
                                  const EdgeInsets.only(left: 30, right: 10),
                              border: OutlineInputBorder(
                                  borderRadius:
                                      BorderRadius.all(Radius.circular(15))),
                            ),
                            hint: Text('state'),
                            value: intState,
                            items: states.map((location) {
                              return new DropdownMenuItem<int>(
                                child: Text(location.name),
                                value: location.id,
                              );
                            }).toList(),
                            onChanged: (int value) {
                              setState(() {
                                intState = value;
                             intDistrict=null;
                              getDistricts();
                              });
                            }),
                      ),
                     Padding(
                        padding: const EdgeInsets.only(top: 15.0),
                        child: DropdownButtonFormField<int>(
                            decoration: InputDecoration(
                              contentPadding:
                                  const EdgeInsets.only(left: 30, right: 10),
                              border: OutlineInputBorder(
                                  borderRadius:
                                      BorderRadius.all(Radius.circular(15))),
                            ),
                            hint: Text(' district'),
                            value: intDistrict,
                            items: districts.map((location) {
                              return new DropdownMenuItem<int>(
                                child: Text(location.name),
                                value: location.id,
                              );
                            }).toList(),
                            onChanged: (int value) {
                              intDistrict = value;
                            }),
                      ),
                    ],
                  ),
                ),
              ),
            ),
          ),
          ProgressLoader(
            loader: loader,
          )
        ],
      ),
    );
  }
}

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