Only static members can be accessed in initalizers

For my project, I need to add a datepicker into my stepper. Unfortunately, Android Studio constantly spits the “Only static members can be accessed in initializers” error out and I’m not able to solve it.
I’m using the flutter_datetime_picker plugin.
Here’s my code:

class DateTimePicker extends StatefulWidget {
  @override
  _DateTimePickerState createState() => _DateTimePickerState();
}

class _DateTimePickerState extends State<DateTimePicker> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: StepperBody(),
    );
  }
}


class StepperBody extends StatefulWidget {
  @override
  _StepperBodyState createState() =>  _StepperBodyState();
}

class _StepperBodyState extends State<StepperBody> {
  String _date = "Not set";
  int currStep = 0;
  static var _focusNode =  FocusNode();
  GlobalKey<FormState> _formKey =  GlobalKey<FormState>();
  static MyData data =  MyData();

  @override
  void initState() {
    super.initState();
    _focusNode.addListener(() {
      setState(() {});
      print('Has focus: $_focusNode.hasFocus');
    });
  }





  List<Step> steps = [
    Step(
        title: const Text('Name',
          style: TextStyle(
            fontFamily: 'Maax',
            fontSize: 20,
          ),),
        //subtitle: const Text('Enter your name'),
        isActive: true,
        //state: StepState.error,
        state: StepState.indexed,
        content: Form(
          key: formKeys[0],
          child: Column(
            children: <Widget>[
              TextFormField(
                style: TextStyle(
                  fontFamily: 'Maax',
                  fontWeight: FontWeight.w500,
                ),
                focusNode: _focusNode,
                keyboardType: TextInputType.text,
                autocorrect: false,
                onSaved: (String value) {
                  data.title = value;
                },
                maxLines: 1,
                //initialValue: 'Aseem Wangoo',
                validator: (value) {
                  if (value.isEmpty || value.length < 1) {
                    return 'Please enter title';
                  }
                },
                decoration:  InputDecoration(
                    labelText: 'What is the name of the test?',
                    hintText: 'Enter a name',
                    //filled: true,
                    icon: const FaIcon(FontAwesomeIcons.signature),
                    labelStyle:
                    TextStyle(decorationStyle: TextDecorationStyle.solid, fontFamily: 'Maax', fontWeight: FontWeight.w500)),
              ),
            ],
          ),
        )),
    Step(
        title: const Text('Date'),
        //subtitle: const Text('Subtitle'),
        isActive: true,
        //state: StepState.editing,
        state: StepState.indexed,
        content: Form(
          key: formKeys[1],
          child: Column(
            children: <Widget>[
              RaisedButton(
                shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(5.0)),
                elevation: 4.0,
                onPressed: () {
                  DatePicker.showDatePicker(context,
                      theme: DatePickerTheme(
                        containerHeight: 210.0,
                      ),
                      showTitleActions: true,
                      minTime: DateTime(2000, 1, 1),
                      maxTime: DateTime(2022, 12, 31), onConfirm: (date) {
                        print('confirm $date');
                        _date = '${date.year} - ${date.month} - ${date.day}';
                        setState(() {});
                      }, currentTime: DateTime.now(), locale: LocaleType.en);
                },
                child: Container(
                  alignment: Alignment.center,
                  height: 50.0,
                  child: Row(
                    mainAxisAlignment: MainAxisAlignment.spaceBetween,
                    children: <Widget>[
                      Row(
                        children: <Widget>[
                          Container(
                            child: Row(
                              children: <Widget>[
                                Icon(
                                  Icons.date_range,
                                  size: 18.0,
                                  color: Colors.teal,
                                ),
                                Text(
                                  " $_date",
                                  style: TextStyle(
                                      color: Colors.teal,
                                      fontWeight: FontWeight.bold,
                                      fontSize: 18.0),
                                ),
                              ],
                            ),
                          )
                        ],
                      ),
                      Text(
                        "  Change",
                        style: TextStyle(
                            color: Colors.teal,
                            fontWeight: FontWeight.bold,
                            fontSize: 18.0),
                      ),
                    ],
                  ),
                ),
                color: Colors.white,
              ),
              TextFormField(
                keyboardType: TextInputType.phone,
                autocorrect: false,
                validator: (value) {
                  if (value.isEmpty || value.length < 10) {
                    return 'Please enter valid number';
                  }
                },
                onSaved: (String value) {
                  data.days = value;
                },
                maxLines: 1,
                decoration:  InputDecoration(
                    labelText: 'Enter your number',
                    hintText: 'Enter a number',
                    icon: const Icon(Icons.phone),
                    labelStyle:
                    TextStyle(decorationStyle: TextDecorationStyle.solid)),
              ),
            ],
          ),
        )),
    Step(
        title: const Text('Email'),
        // subtitle: const Text('Subtitle'),
        isActive: true,
        state: StepState.indexed,
        // state: StepState.disabled,
        content:  Form(
          key: formKeys[2],
          child: Column(
            children: <Widget>[
              TextFormField(
                keyboardType: TextInputType.emailAddress,
                autocorrect: false,
                validator: (value) {
                  if (value.isEmpty || !value.contains('@')) {
                    return 'Please enter valid email';
                  }
                },
                onSaved: (String value) {
                  data.words = value;
                },
                maxLines: 1,
                decoration:  InputDecoration(
                    labelText: 'Enter your email',
                    hintText: 'Enter a email address',
                    icon: const Icon(Icons.email),
                    labelStyle:
                    TextStyle(decorationStyle: TextDecorationStyle.solid)),
              ),
            ],
          ),
        )),
    Step(
        title: const Text('Age'),
        // subtitle: const Text('Subtitle'),
        isActive: true,
        state: StepState.indexed,
        content:  Form(
          key: formKeys[3],
          child: Column(
            children: <Widget>[
              TextFormField(
                keyboardType: TextInputType.number,
                autocorrect: false,
                validator: (value) {
                  if (value.isEmpty || value.length > 2) {
                    return 'Please enter valid age';
                  }
                },
                maxLines: 1,
                onSaved: (String value) {
                  data.rep = value;
                },
                decoration:  InputDecoration(
                    labelText: 'Enter your age',
                    hintText: 'Enter age',
                    icon: const Icon(Icons.explicit),
                    labelStyle:
                    TextStyle(decorationStyle: TextDecorationStyle.solid)),
              ),
            ],
          ),
        )),
    //  Step(
    //     title: const Text('Fifth Step'),
    //     subtitle: const Text('Subtitle'),
    //     isActive: true,
    //     state: StepState.complete,
    //     content: const Text('Enjoy Step Fifth'))
  ];

  @override
  Widget build(BuildContext context) {
    void showSnackBarMessage(String message,
        [MaterialColor color = Colors.red]) {
      Scaffold
          .of(context)
          .showSnackBar( SnackBar(content:  Text(message)));
    }

    void _submitDetails() {
      final FormState formState = _formKey.currentState;

      if (!formState.validate()) {
        showSnackBarMessage('Please enter correct data');
      } else {
        formState.save();
        print("Name: ${data.title}");
        print("Phone: ${data.days}");
        print("Email: ${data.words}");
        print("Age: ${data.rep}");

        showDialog(
            context: context,
            child:  AlertDialog(
              title:  Text("Details"),
              //content:  Text("Hello World"),
              content:  SingleChildScrollView(
                child:  ListBody(
                  children: <Widget>[
                    Text("Name : " + data.title),
                    Text("Phone : " + data.days),
                    Text("Email : " + data.words),
                    Text("Age : " + data.rep),
                  ],
                ),
              ),
              actions: <Widget>[
                FlatButton(
                  child:  Text('OK'),
                  onPressed: () {
                    Navigator.of(context).pop();
                  },
                ),
              ],
            ));
      }
    }

    return  Container(
        child:  Form(
          key: _formKey,
          child:  ListView(children: <Widget>[
            Stepper(
              steps: steps,
              type: StepperType.vertical,
              currentStep: this.currStep,
              onStepContinue: () {
                setState(() {
                  if(formKeys[currStep].currentState.validate()) {
                    if (currStep < steps.length - 1) {
                      currStep = currStep + 1;
                    } else {
                      currStep = 0;
                    }
                  }
                  // else {
                  // Scaffold
                  //     .of(context)
                  //     .showSnackBar( SnackBar(content:  Text('$currStep')));

                  // if (currStep == 1) {
                  //   print('First Step');
                  //   print('object' + FocusScope.of(context).toStringDeep());
                  // }

                  // }
                });
              },
              onStepCancel: () {
                setState(() {
                  if (currStep > 0) {
                    currStep = currStep - 1;
                  } else {
                    currStep = 0;
                  }
                });
              },
              onStepTapped: (step) {
                setState(() {
                  currStep = step;
                });
              },
            ),
            RaisedButton(
              child:  Text(
                'Save details',
                style:  TextStyle(color: Colors.white),
              ),
              onPressed: _submitDetails,
              color: Colors.blue,
            ),

          ]),
        ));
  }
}

Any help is appreciated.

@Gandalf_Doner For which value is this error coming? Can you please share the error snippet as well?