在构建 LoadDataFromFireStore 时抛出了以下 NoSuchMethodError。在 null 上调用了 getter 'keys'

Question

我想连接到我的 firebase 数据库，现在称为“实时数据库”。 我的应用程序运行顺利，但如果我想通过按钮在我的应用程序中启动该方法，我会收到以下错误：

======== Exception caught by widgets library =======================================================
The following NoSuchMethodError was thrown building LoadDataFromFireStore(dirty, state: LoadDataFromFireStoreState#20451):
The getter 'keys' was called on null.
Receiver: null
Tried calling: keys

The relevant error-causing widget was: 
  LoadDataFromFireStore file:///C:/Users/Nutzer/AndroidStudioProjects/tennis_sv_schwaig/lib/widget/kalender.dart:17:13
When the exception was thrown, this was the stack: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      LoadDataFromFireStoreState._showCalendar (package:tennis_sv_schwaig/widget/kalender.dart:58:34)
#2      LoadDataFromFireStoreState.build (package:tennis_sv_schwaig/widget/kalender.dart:49:13)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4744:28)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4627:15)
...
====================================================================================================

我不知道之前在其他方法中是否与 keyForm 发生冲突。这可能吗？ 这是该方法的代码：

import 'dart:math';
import 'dart:collection';
import 'package:flutter/material.dart';
import 'package:syncfusion_flutter_calendar/calendar.dart';
import 'package:intl/intl.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:cloud_firestore/cloud_firestore.dart';



void main() => runApp(kalender());

class kalender extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: LoadDataFromFireStore(),
    );
  }
}

class LoadDataFromFireStore extends StatefulWidget {
  @override
  LoadDataFromFireStoreState createState() => LoadDataFromFireStoreState();
}

class LoadDataFromFireStoreState extends State<LoadDataFromFireStore> {
  DataSnapshot querySnapshot;
  dynamic data;
  List<Color> _colorCollection;

  @override
  void initState() {
    _initializeEventColor();
    getDataFromDatabase().then((results) {
      setState(() {
        if (results != null) {
          querySnapshot = results;
        }
      });
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(

      body: _showCalendar(),
    );
  }

  _showCalendar() {
    if (querySnapshot != null) {
      List<Meeting> collection;
      var showData = querySnapshot.value;
      Map<dynamic, dynamic> values = showData;
      List<dynamic> key = values.keys.toList();
      if (values != null) {
        for (int i = 0; i < key.length; i++) {
          data = values[key[i]];
          collection ??= <Meeting>[];
          final Random random = new Random();
          collection.add(Meeting(
              eventName: data['Subject'],
              isAllDay: false,
              from: DateFormat('dd/MM/yyyy HH:mm:ss').parse(data['StartTime']),
              to: DateFormat('dd/MM/yyyy HH:mm:ss').parse(data['EndTime']),
              background: _colorCollection[random.nextInt(9)]));
        }
      } else {
        return Center(
          child: CircularProgressIndicator(),
        );
      }

      return SafeArea(
          child: Column(
            children: [
              Container(
                height: 400,
                child: SfCalendar(
                  view: CalendarView.month,
                  initialDisplayDate: DateTime(2020, 4, 5, 9, 0, 0),
                  dataSource: _getCalendarDataSource(collection),
                  monthViewSettings: MonthViewSettings(showAgenda: true),
                ),
              ),
              RaisedButton(onPressed: () {
                final dbRef = FirebaseDatabase.instance.reference().child("CalendarData");
                dbRef.push().set({
                  "StartTime": '07/04/2020 07:00:00',
                  "EndTime": '07/04/2020 08:00:00',
                  "Subject":'NewMeeting',
                }).then((_) {
                  Scaffold.of(context).showSnackBar(
                      SnackBar(content: Text('Successfully Added')));
                }).catchError((onError) {
                  print(onError);
                });
              }, child: Text("Add")),
              RaisedButton(onPressed: () {
                final dbRef = FirebaseDatabase.instance.reference().child("CalendarData");
                dbRef.remove();
              }, child: Text("Delete")),
            ],
          ));
    }
  }

  void _initializeEventColor() {
    this._colorCollection = new List<Color>();
    _colorCollection.add(const Color(0xFF0F8644));
    _colorCollection.add(const Color(0xFF8B1FA9));
    _colorCollection.add(const Color(0xFFD20100));
    _colorCollection.add(const Color(0xFFFC571D));
    _colorCollection.add(const Color(0xFF36B37B));
    _colorCollection.add(const Color(0xFF01A1EF));
    _colorCollection.add(const Color(0xFF3D4FB5));
    _colorCollection.add(const Color(0xFFE47C73));
    _colorCollection.add(const Color(0xFF636363));
    _colorCollection.add(const Color(0xFF0A8043));
  }
}

MeetingDataSource _getCalendarDataSource([List<Meeting> collection]) {
  List<Meeting> meetings = collection ?? <Meeting>[];
  return MeetingDataSource(meetings);
}

class MeetingDataSource extends CalendarDataSource {
  MeetingDataSource(List<Meeting> source) {
    appointments = source;
  }

  @override
  DateTime getStartTime(int index) {
    return appointments[index].from;
  }

  @override
  DateTime getEndTime(int index) {
    return appointments[index].to;
  }

  @override
  bool isAllDay(int index) {
    return appointments[index].isAllDay;
  }

  @override
  String getSubject(int index) {
    return appointments[index].eventName;
  }

  @override
  Color getColor(int index) {
    return appointments[index].background;
  }
}

getDataFromDatabase() async {
  var value = FirebaseDatabase.instance.reference();
  var getValue = await value.child('CalendarData').once();
  return getValue;
}

class Meeting {
  Meeting({this.eventName, this.from, this.to, this.background, this.isAllDay});

  String eventName;
  DateTime from;
  DateTime to;
  Color background;
  bool isAllDay;
}

希望它足够了，任何人都可以提供帮助。非常感谢！