OnPressed（）返回方法“ call”在null上被调用。接收方：null尝试调用：call（）为错误

Question

我正在创建一个按钮，当按下该按钮时，它将切换到另一个视图，我正在使用3个不同的文件，一个包含所有功能的root_page.dart和2个dart文件（doctor_list.dart和doctor_map。 dart），其中包含2个不同的视图。但是我按下按钮，我收到此错误消息“在null上调用了'call'方法。接收者：null尝试调用：call（）”。我该如何解决？

root_page.dart

class RootPage extends StatefulWidget {
  @override
  _RootPageState createState() => _RootPageState();
}

enum Views {
  doctorList,
  doctorMap
}


class _RootPageState extends State<RootPage> {
  Views _currentView;

  void initState() {
    super.initState();
    setState(() {
      _currentView = Views.doctorMap;
    });
  }

  void _toDoctorListView() {
    setState(() {
      _currentView = Views.doctorList;
    });
  }

  void _toDoctorMapView() {
    setState(() {
      _currentView = Views.doctorMap;
    });
  }

  @override
  Widget build(BuildContext context) {
    if(_currentView == Views.doctorList) {
      return DoctorListView(
        toDoctorMapView: _toDoctorMapView,
      );
    } else if (_currentView == Views.doctorMap) {
      return DoctorMapView(
        toDoctorListView: _toDoctorListView,
      );
    } else {
      return DoctorMapView(
        toDoctorListView: _toDoctorListView,
      );
    }
  }
}

doctor_map.dart

class DoctorMapView extends StatefulWidget {
  DoctorMapView({this.toDoctorListView});
  final VoidCallback toDoctorListView;

  @override
  _DoctorMapViewState createState() => _DoctorMapViewState();
}

class _DoctorMapViewState extends State<DoctorMapView> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
          appBar: AppBar(
            backgroundColor: Constants.APPBAR_COLOR,
            title: Text("Hello"),
            ),
            leading: IconButton(
              icon: Icon(
                Icons.arrow_back_ios,
                color: Colors.white,
              ),
              onPressed: () {
                Navigator.pop(context);
              },
            ),
            actions: <Widget>[
              IconButton(
                icon: Icon(Icons.swap_horiz),
                onPressed: () {
                  widget.toDoctorListView();
                },
              )
            ],
          ),
      ),
    );
  }
}

doctor_list.dart

class DoctorListView extends StatefulWidget {
  DoctorListView({this.toDoctorMapView});
  final VoidCallback toDoctorMapView;

  @override
  _DoctorListViewState createState() => _DoctorListViewState();
}

class _DoctorListViewState extends State<DoctorListView> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          backgroundColor: Constants.APPBAR_COLOR,
          elevation: 1.0,
          title: Text(
            "Pediatricians",
            style: Constants.APPBAR_TITLE,
          ),
          leading: IconButton(
            icon: Icon(
              Icons.arrow_back_ios,
              size: 22.5,
              color: Constants.LIGHT_COLOR,
            ),
            onPressed: () {
              Navigator.pop(context);
            },
          ),
          actions: <Widget>[
            IconButton(
              icon: Icon(Icons.swap_horiz),
              onPressed: () {
                widget.toDoctorMapView();
              },
            )
          ],
        ),
        body: ChatListContainer(),
      ),
    );
  }
}