Flutter - состояние было вызвано нулем во время GestureDetector

Question

У меня есть StateFul виджет и class с конструктором. Этот класс имеет functions, и каждая функция просто возвращает widget. Я хочу получить доступ к состоянию виджета Stateful изнутри функции другого class. Вот почему я использую context.findAncestorStateOfType<>().

class MyHomePage extends StatefulWidget {

  static MyHomePageState of(BuildContext context) => context.findAncestorStateOfType<MyHomePageState>();

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

Внутри initState() виджета Stateful я инициализирую конструктор другого class с параметрами. Одна из функций имеет GestureDetector для виджета. Внутри onTap() GestureDetector я пытался вызвать изменение значения переменной в виджете Stateful. Это где я получаю нулевую ошибку. Я попытался установить конструктор из build(), но я получаю ту же ошибку. Пожалуйста, помогите мне с этим. Заранее спасибо.

Журнал ошибок :

The following NoSuchMethodError was thrown while handling a gesture:
The setter 'choosingDestinationFromMap=' was called on null.
Receiver: null
Tried calling: choosingDestinationFromMap=true

When the exception was thrown, this was the stack: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1      MapHelper.destinationPointWidget.<anonymous closure> (package:google_map/map_helper.dart:511:42)
#2      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
#3      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:486:11)
#4      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:264:5)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#30707
  debugOwner: GestureDetector
  state: possible
  won arena
  finalPosition: Offset(140.4, 170.2)
  finalLocalPosition: Offset(140.4, 23.1)
  button: 1
  sent tap down

initState () :

@override
  void initState() {

    super.initState();

    controller1 =
        AnimationController(vsync: this, duration: Duration(milliseconds: 200));
    controller2 =
        AnimationController(vsync: this, duration: Duration(milliseconds: 150));
    controller2_1 =
        AnimationController(vsync: this, duration: Duration(milliseconds: 500));
    controller3 =
        AnimationController(vsync: this, duration: Duration(milliseconds: 150));
    controller3_1 =
        AnimationController(vsync: this, duration: Duration(milliseconds: 500));

    offset1 = Tween<Offset>(begin: Offset(0.0, -1.0), end: Offset.zero).animate(controller1);
    offset2 = Tween<Offset>(begin: Offset(0.0, -1.0), end: Offset.zero).animate(controller2);
    offset2_1 = Tween<Offset>(begin: Offset(0.0, -0.40), end: Offset.zero).animate(controller2_1);
    offset3 = Tween<Offset>(begin: Offset(0.0, -1.0), end: Offset.zero).animate(controller3);
    offset3_1 = Tween<Offset>(begin: Offset(0.0, -0.40), end: Offset.zero).animate(controller3_1);

    mapHelper = MapHelper(controller1, controller2, controller2_1, controller3,
        controller3_1, offset1, offset2, offset2_1, offset3,
        offset3_1, textController1, textController2, context);
}

Другой класс :

class MapHelper {

  AnimationController controller1;
  AnimationController controller2;
  AnimationController controller2_1;
  AnimationController controller3;
  AnimationController controller3_1;

  Animation<Offset> offset1;
  Animation<Offset> offset2;
  Animation<Offset> offset2_1;
  Animation<Offset> offset3;
  Animation<Offset> offset3_1;

  TextEditingController textController1;
  TextEditingController textController2;

  BuildContext context;

  MapHelper(this.controller1, this.controller2, this.controller2_1,
      this.controller3, this.controller3_1, this.offset1, this.offset2,
      this.offset2_1, this.offset3, this.offset3_1, this.textController1,
      this.textController2, this.context);


  destinationPointWidget() {

    return SlideTransition(
      position: offset3,
      child: Align(
        alignment: Alignment.topCenter,
        child: Container(
          color: Colors.white,
          height: 110,
          width: double.infinity,
          child: Column(
            mainAxisAlignment: MainAxisAlignment.end,
            children: <Widget>[

              SlideTransition(
                position: offset3_1,
                child: Align(
                  alignment: Alignment.bottomCenter,
                  child: Container(
                    alignment: Alignment.center,
                    margin: EdgeInsets.only(left: 8, right: 8),
                    decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(10),
                      border: Border.all(color: Colors.black26),
                    ),
                    child: Row(
                      children: <Widget>[

                        Flexible(
                          flex: 1,
                          child: IconButton(
                            icon: Icon(
                              Icons.arrow_back,
                              color: Colors.black,
                            ),
                            onPressed: () {
                              controller3.reverse();
                              controller3_1.reverse();
                              controller1.forward();
                            },
                          ),
                        ),

                        SizedBox(width: 10,),

                        Flexible(
                          flex: 7,
                          child: TextField(
                            controller: textController1,
                            cursorRadius: Radius.circular(10),
                            style: TextStyle(
                              fontSize: 17,
                            ),
                            decoration: InputDecoration(
                              hintText: "Choose destination point",
                              border: InputBorder.none,
                            ),
                            onSubmitted: null,
                          ),
                        ),
                      ],
                    ),
                  ),
                ),
              ),

              SizedBox(height: 6,),

              GestureDetector(
                child: Container(
                  padding: EdgeInsets.only(bottom: 8),
                  margin: EdgeInsets.only(left: 8),
                  child: Row(
                    children: <Widget>[

                      Flexible(
                        flex: 1,
                        child: Container(
                          alignment: Alignment.center,
                          height: 40,
                          child: CircleAvatar(
                            radius: 15,
                            backgroundColor: Colors.black12,
                            child: Icon(Icons.map, color: Colors.black54, size: 20,),
                          ),
                        ),
                      ),

                      SizedBox(width: 10,),

                      Flexible(
                        flex: 8,
                        child: Container(
                          height: 40,
                          alignment: Alignment.centerLeft,
                          child: Text(
                            "Choose on map",
                            style: TextStyle(
                              fontSize: 16,
                            ),
                          ),
                        ),
                      ),
                    ],
                  ),
                ),
                onTap: () {

                  MyHomePage.of(context).choosingDestinationFromMap = true;
                  print(MyHomePage.of(context).choosingDestinationFromMap);
                  MyHomePage.of(context).positionMarkerAtCenter();
                },
              ),
            ],
          ),
        ),
      ),
    );
  }
}