The method '[]' was called on null. Receiver: null Tried calling: [](Instance of 'DateTime')

Question

I have a calendar in my project and I have a button to add events to dates in that calendar. When I am trying to press the add button to add an event I am getting The method '[]' was called on null. Receiver: null Tried calling: [](Instance of 'DateTime'). Something is returning null but I can not understand what. Can you help me?

Here are my codes:

calendar.dart

import 'package:flutter_calendar_carousel/classes/event.dart';
import 'package:flutter_calendar_carousel/flutter_calendar_carousel.dart';
import 'package:intl/intl.dart';

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  DateTime _currentDate = DateTime.now();
  DateTime _currentDate2 = DateTime.now();
  String _currentMonth = DateFormat.yMMM().format(DateTime.now());
  DateTime _targetDateTime = DateTime.now();

  Map<DateTime, List<Event>> selectedEvents;
  DateTime selectedDay = DateTime.now();
  DateTime focusedDay = DateTime.now();

  TextEditingController _eventController = TextEditingController();

  CalendarCarousel _calendarCarouselNoHeader;

  static Widget _eventIcon = new Container(
    decoration: new BoxDecoration(
        color: Colors.white,
        borderRadius: BorderRadius.all(Radius.circular(1000)),
        border: Border.all(color: Colors.blue, width: 2.0)),
    child: new Icon(
      Icons.person,
      color: Colors.amber,
    ),
  );

  EventList<Event> _markedDateMap = new EventList<Event>(
    events: {
      new DateTime(2020, 2, 10): [
        new Event(
          date: new DateTime(2020, 2, 14),
          title: 'Event 1',
          icon: _eventIcon,
          dot: Container(
            margin: EdgeInsets.symmetric(horizontal: 1.0),
            color: Colors.red,
            height: 5.0,
            width: 5.0,
          ),
        ),
        new Event(
          date: new DateTime(2020, 2, 10),
          title: 'Event 2',
          icon: _eventIcon,
        ),
        new Event(
          date: new DateTime(2020, 2, 15),
          title: 'Event 3',
          icon: _eventIcon,
        ),
      ],
    },
  );

  @override
  void initState() {
    _markedDateMap.add(
        new DateTime(2020, 2, 25),
        new Event(
          date: new DateTime(2020, 2, 25),
          title: 'Event 5',
          icon: _eventIcon,
        ));

    _markedDateMap.add(
        new DateTime(2020, 2, 10),
        new Event(
          date: new DateTime(2020, 2, 10),
          title: 'Event 4',
          icon: _eventIcon,
        ));

    _markedDateMap.addAll(new DateTime(2019, 2, 11), [
      new Event(
        date: new DateTime(2019, 2, 11),
        title: 'Event 1',
        icon: _eventIcon,
      ),
      new Event(
        date: new DateTime(2019, 2, 11),
        title: 'Event 2',
        icon: _eventIcon,
      ),
      new Event(
        date: new DateTime(2019, 2, 11),
        title: 'Event 3',
        icon: _eventIcon,
      ),
    ]);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    _calendarCarouselNoHeader = CalendarCarousel<Event>(
      todayBorderColor: Color(0xFF3EB16F),
      onDayPressed: (DateTime date, List<Event> events) {
        this.setState(() => _currentDate2 = date);
        events.forEach((event) => print(event.title));
      },
      daysHaveCircularBorder: true,
      showOnlyCurrentMonthDate: false,
      weekendTextStyle: TextStyle(
        color: Colors.red,
      ),
      thisMonthDayBorderColor: Colors.grey,
      weekFormat: false,
//      firstDayOfWeek: 4,
      markedDatesMap: _markedDateMap,
      height: 420.0,
      selectedDateTime: _currentDate2,
      targetDateTime: _targetDateTime,
      customGridViewPhysics: NeverScrollableScrollPhysics(),
      markedDateCustomShapeBorder:
          CircleBorder(side: BorderSide(color: Colors.yellow)),
      markedDateCustomTextStyle: TextStyle(
        fontSize: 18,
        color: Colors.blue,
      ),
      showHeader: false,
      todayTextStyle: TextStyle(
        color: Colors.blue,
      ),

      todayButtonColor: Colors.yellow,
      selectedDayTextStyle: TextStyle(
        color: Colors.yellow,
      ),
      minSelectedDate: _currentDate.subtract(Duration(days: 360)),
      maxSelectedDate: _currentDate.add(Duration(days: 360)),
      prevDaysTextStyle: TextStyle(
        fontSize: 16,
        color: Colors.pinkAccent,
      ),
      inactiveDaysTextStyle: TextStyle(
        color: Colors.tealAccent,
        fontSize: 16,
      ),
      onCalendarChanged: (DateTime date) {
        this.setState(() {
          _targetDateTime = date;
          _currentMonth = DateFormat.yMMM().format(_targetDateTime);
        });
      },
      onDayLongPressed: (DateTime date) {
        print('long pressed date $date');
      },
    );

    return new Scaffold(
        appBar: AppBar(
          toolbarHeight: 90,
          backgroundColor: Color(0xFF3EB16F),
          title: Text("Calendar"),
          centerTitle: true,
          textTheme: TextTheme(
            headline6: TextStyle(fontSize: 44, fontFamily: "Angel"),
          ),
        ),
        body: SingleChildScrollView(
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              //custom icon

              Container(
                margin: EdgeInsets.only(
                  top: 30.0,
                  bottom: 16.0,
                  left: 16.0,
                  right: 16.0,
                ),
                child: new Row(
                  children: <Widget>[
                    Expanded(
                        child: Text(
                      _currentMonth,
                      style: TextStyle(
                        fontWeight: FontWeight.bold,
                        fontSize: 24.0,
                      ),
                    )),
                    FlatButton(
                      child: Text('PREV'),
                      onPressed: () {
                        setState(() {
                          _targetDateTime = DateTime(
                              _targetDateTime.year, _targetDateTime.month - 1);
                          _currentMonth =
                              DateFormat.yMMM().format(_targetDateTime);
                        });
                      },
                    ),
                    FlatButton(
                      child: Text('NEXT'),
                      onPressed: () {
                        setState(() {
                          _targetDateTime = DateTime(
                              _targetDateTime.year, _targetDateTime.month + 1);
                          _currentMonth =
                              DateFormat.yMMM().format(_targetDateTime);
                        });
                      },
                    )
                  ],
                ),
              ),
              Container(
                margin: EdgeInsets.symmetric(horizontal: 16.0),
                child: _calendarCarouselNoHeader,
              ), //
            ],
          ),
        ),
        floatingActionButton: FloatingActionButton.extended(
          onPressed: () => showDialog(
            context: context,
            builder: (context) => AlertDialog(
              title: Text("Add Event"),
              content: TextFormField(
                controller: _eventController,
              ),
              actions: [
                TextButton(
                  onPressed: () => Navigator.pop(context),
                  child: Text("Cancel"),
                ),
                TextButton(
                  child: Text("Add"),
                  onPressed: () {
                    if (_eventController.text.isEmpty) {
                    } else {
                      if (selectedEvents[selectedDay] != null) {
                        selectedEvents[selectedDay].add(
                          Event(title: _eventController.text),
                        );
                      } else {
                        selectedEvents[selectedDay] = [
                          Event(title: _eventController.text)
                        ];
                      }
                    }
                    Navigator.pop(context);
                    _eventController.clear();
                    setState(() {});
                    return;
                  },
                ),
              ],
            ),
          ),
          label: Text("Add"),
          icon: Icon(Icons.add),
        ));
  }
}

event.dart

class Event {
  final String title;
  Event({@required this.title});

  String toString() => this.title;
}

Answer 1

Try: Map<DateTime, List<Event>> selectedEvents = Map();

Answer 2

The selectedEvents variable is null, hence you get this error when trying to access the value by key. What you need to do to avoid that is initiating this variable with empty map:

Map<DateTime, List<Event>> selectedEvents = <DateTime, List<Event>>{};

Also, instead of checking if the value is null, you can use Map methods for that, e.g.:

Instead of calling this:

if (selectedEvents[selectedDay] != null) {...}

Use a dedicated method:

if (selectedEvents.containsKey(selectedDay)) {...}