方法'call'在null上被调用

Question

我目前正在开发一个应用程序，该程序需要从用户那里获取日期时间并进行存储。 我正在使用一个将datetime从子代传递到父代的回调，但始终收到以下错误。

在DateTimePicker类中，onPressed执行callBack函数，它将日期传递给其父级，这可能是我的问题所在。

我是个新手，我认为我的代码有点混乱，但是有些帮助将不胜感激。

     The following NoSuchMethodError was thrown while handling a gesture:
     The method 'call' was called on null.
     Receiver: null
     Tried calling: call(Instance of 'DateTime')

子类的代码（我认为是问题所在）

     import 'package:flutter/material.dart';

    import 'package:flutter_datetime_picker/flutter_datetime_picker.dart';
    import 'package:intl/intl.dart';

    class DateTimePicker extends StatelessWidget {

      final Function callBack;

      const DateTimePicker({Key key, this.callBack}) : super(key: key);

      @override
      Widget build(BuildContext context) {
        String _date = "Not set";
        return Container(
          padding: EdgeInsets.only(left: 15, right: 15),
          child: Column(
            children: <Widget>[
              RaisedButton(
                shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(8.0),
                ),
                elevation: 4.0,
                onPressed: () {
                  DatePicker.showDatePicker(context,
                      theme: DatePickerTheme(
                        containerHeight: 250.0,
                        cancelStyle: TextStyle(
                            fontFamily: 'Lato', fontSize: 22, color: Colors.red),
                        doneStyle: TextStyle(
                            fontFamily: 'Lato', fontSize: 22, color: Colors.blue),
                        itemStyle: TextStyle(
                            fontFamily: 'Montserrat',
                            fontSize: 20,
                            color: Colors.black),
                        itemHeight: 50,
                      ),
                      showTitleActions: true,
                      minTime: DateTime(2000, 1, 1),
                      maxTime: DateTime(2022, 12, 31), onConfirm: (date) {
                    print('confirm $date');
                    callBack(date);
                    _date = DateFormat.yMMMd().format(date);
                  }, currentTime: DateTime.now(), locale: LocaleType.en);
                },
                child: Container(
                  alignment: Alignment.center,
                  height: 65,
                  child: Row(
                    mainAxisAlignment: MainAxisAlignment.spaceBetween,
                    children: <Widget>[
                      Row(
                        children: <Widget>[
                          Container(
                            child: Row(
                              children: <Widget>[
                                Icon(
                                  Icons.calendar_today,
                                  size: 28.0,
                                  color: Colors.white,
                                ),
                                SizedBox(
                                  width: 20,
                                ),
                                Text(
                                  " $_date",
                                  style: TextStyle(
                                      color: Colors.white,
                                      fontSize: 22.0,
                                      fontFamily: 'Lato'),
                                ),
                              ],
                            ),
                          )
                        ],
                      ),
                      Text(
                        "  Change",
                        style: TextStyle(
                            color: Colors.white,
                            fontSize: 20.0,
                            fontFamily: 'Lato'),
                      ),
                    ],
                  ),
                ),
                color: Color.fromRGBO(79, 67, 67, 1),
              )
            ],
          ),
        );
      }
    }

父类的代码

  import 'package:flutter/material.dart';

    import '../widgets/date_picker.dart';
    import 'package:provider/provider.dart';
    import '../providers/event_provider.dart';

    class EventPicker extends StatefulWidget {
      @override
      _EventPickerState createState() => _EventPickerState();
    }

    class _EventPickerState extends State<EventPicker> {
      String eventName;

      String dateTime;

      void callBack(DateTime dateFromChild) {
        this.dateTime = dateFromChild.toIso8601String();
      }

      @override
      Widget build(BuildContext context) {
        final add = Provider.of<EventProviders>(context);

        return SingleChildScrollView(
          child: Center(
            child: Container(
              child: Column(
                children: <Widget>[
                  Padding(
                    padding: const EdgeInsets.only(top: 40, bottom: 60.0),
                    child: Text(
                      'Add Events',
                      style: Theme.of(context).textTheme.headline,
                    ),
                  ),
                  DateTimePicker(),
                  SizedBox(height: 27),
                  Theme(
                    data: ThemeData(
                      primaryColor: Colors.grey,
                      primaryColorDark: Colors.red,
                      fontFamily: 'Lato',
                    ),
                    child: Padding(
                      padding: const EdgeInsets.only(left: 15.0, right: 15.0),
                      child: TextField(
                        onChanged: (value) {
                          eventName = value;
                        },
                        autofocus: true,
                        decoration: InputDecoration(
                          border: OutlineInputBorder(),
                          labelText: 'Event',
                          labelStyle: TextStyle(
                            color: Colors.white,
                            fontFamily: 'Montserrat',
                          ),
                          hintText: 'eg: Exam ',
                          hintStyle: TextStyle(color: Colors.white, fontSize: 20)
                              .copyWith(fontFamily: 'Lato'),
                        ),
                        maxLength: 20,
                        style: TextStyle(color: Colors.white, fontSize: 22),
                        cursorColor: Colors.white,
                      ),
                    ),
                  ),
                  SizedBox(
                    height: 10,
                  ),
                  IconButton(
                    onPressed: () => add.addEvent(eventName, dateTime),
                    icon: Icon(
                      Icons.save,
                      color: Colors.white,
                      size: 30,
                    ),
                  ),
                  Text(
                    'Save',
                    style: Theme.of(context).textTheme.body2,
                  ),
                ],
              ),
            ),
          ),
        );
      }
    }