问未处理的异常:键入'_DropdownRouteResult<int>‘不是'int?’类型的子类型。从Navigator.pop()返回值时

EN

E/flutter (24895): [ERROR:flutter/lib/ui/ui_dart_state.cc(198)] Unhandled Exception: type '_DropdownRouteResult<int>' is not a subtype of type 'int?'
E/flutter (24895): #0      _TeaProducerFormFieldState.build.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:teavault/screens/stash/teaform.dart:70:15)
E/flutter (24895): #1      State.setState (package:flutter/src/widgets/framework.dart:1109:30)
E/flutter (24895): #2      _TeaProducerFormFieldState.build.<anonymous closure>.<anonymous closure> (package:teavault/screens/stash/teaform.dart:69:13)
E/flutter (24895): #3      _rootRunUnary (dart:async/zone.dart:1434:47)
E/flutter (24895): #4      _CustomZone.runUnary (dart:async/zone.dart:1335:19)

class _TeaProducerFormFieldState extends State<TeaProducerFormField> {
  int? _selectedValue;

  @override
  Widget build(BuildContext context) {

    final teaProducers = Provider.of<TeaProducerCollectionModel>(context).items;
    final teaProducerListItems = teaProducers.map((producer) => DropdownMenuItem<int>(
      value: producer.id,
      child: Text(producer.name),
    )).toList();

    final workingAddNewTeaProducerButton = DropdownMenuItem<int>(child: TextButton(
        onPressed: () async {
          final int newValue = await Navigator.push(context, AddNewTeaProducerRoute());
          setState(() {
            _selectedValue = newValue;
          });
        },
        child: Text('Add New Manufacturer')));

    final brokenAddNewTeaProducerButtion = DropdownMenuItem<int>(
        onTap: () {
          Navigator.push(context, AddNewTeaProducerRoute()).then((value) {
            setState(() {
              _selectedValue = value;
            });
          });
        },
        value: 0,
        child: const Text('Add New Manufacturer'));

    return DropdownButtonFormField(
      value: _selectedValue,
      items: teaProducerListItems + [brokenAddNewTeaProducerButtion],
      // child: addNewTeaProducerButton)],
      onChanged: (_) {},
      hint: Text('Select a manufacturer'),
    );
  }

}

When the exception was thrown, this was the stack: 
#0      LocalHistoryRoute.didPop (package:flutter/src/widgets/routes.dart)
#1      _RouteEntry.handlePop (package:flutter/src/widgets/navigator.dart:2896:16)
#2      NavigatorState._flushHistoryUpdates (package:flutter/src/widgets/navigator.dart:3868:22)
#3      NavigatorState.pop (package:flutter/src/widgets/navigator.dart:4910:7)
#4      Navigator.pop (package:flutter/src/widgets/navigator.dart:2432:27)
#5      _DropdownMenuItemButtonState._handleOnTap (package:flutter/src/material/dropdown.dart:148:15)

final brokenAddNewTeaProducerButton = DropdownMenuItem<int>(
        onTap: () {
          Navigator.push<int?>(context, MaterialPageRoute(
              builder: (BuildContext context) =>
                  Scaffold(
                      appBar: AppBar(
                        iconTheme: IconThemeData(color: Colors.white),
                        title: Text('Define new Manufacturer'),
                      ),
                      body: Column(
                        children: <Widget>[
                          Expanded(
                            flex: MediaQuery.of(context).orientation == Orientation.portrait ? 4 : 1,
                            child: Container(),
                          ),
                          Expanded(
                            flex: 16,
                            child: Container(
                              padding: const EdgeInsets.symmetric(horizontal: 50.0),
                              child: const TeaProducerForm(),
                            ),
                          ),
                        ],
                      )
                  ))).then((value) {
            setState(() {
              _selectedValue = value;
            });
          });
        },
        value: 0,
        child: const Text('Add New Manufacturer'));

//[...]

class TeaProducerFormState extends State<TeaProducerForm> {
  final _formKey = GlobalKey<FormState>();

  String? fullName;
  String? shortName;

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      child: Column(
        children: <Widget>[
          TextFormField(
            onSaved: ((value) {fullName = value;}),
            decoration: const InputDecoration(
              hintText: 'Enter full name',
              labelText: 'Full Name'
            ),
          ),
          TextFormField(
            onSaved: ((value) {shortName = value;}),
            decoration: const InputDecoration(
                hintText: 'Enter abbreviated name',
                labelText: 'Short Name'
            ),
            validator: ((value) {
              if ((value ?? '').length > 6 ) {
                return 'Please enter a name with six or fewer characters';
              } else if ((value ?? '').isEmpty ) {
                return 'Please enter a name with one or more characters';
              }
            }),
          ),
          ElevatedButton(
              onPressed: () async {
                if (_formKey.currentState?.validate() ?? false) {

                  ScaffoldMessenger.of(context).showSnackBar(const SnackBar(content: Text('Saving new manufacturer...')),);
                  _formKey.currentState?.save();

                  final teaProducers = Provider.of<TeaProducerCollectionModel>(context, listen: false);
                  teaProducers.insert(TeaProducer(
                      name: fullName!,
                      shortName: shortName!
                  )).then((value) {
                    print('added tea as id $value');
                    ScaffoldMessenger.of(context).clearSnackBars();
                    Navigator.pop<int?>(context, value);
                  });
                }
            },
            child: const Text('Submit'))
          // Add TextFormFields and ElevatedButton here.
        ],
      ),
    );
  }
}