Event markers not loading into Table Calendar widget [Flutter]

Question

I'm trying to populate my Table Calendar widget with events from Firebase. I was able to do this successfully, with events being loaded into a list of events associated with that day. While the list shows successfully once I tap on a day, the marker on the calendar showing that the day has an event isn't showing up. Here is some of my code:

  @override
  void initState() {
    _groupEvents();
    super.initState();
  }

  List<PostEvent> _getEventsForDay(DateTime day) {
    if (_selectedEvents[day] != null) {
      return _selectedEvents[day]!;
    }
    return [];
  }

  Map<DateTime, List<PostEvent>> _groupEvents() {
    for (var event in postedEvents) {
      DateTime date = DateTime(event.dateTimeOfEvent.year,
          event.dateTimeOfEvent.month, event.dateTimeOfEvent.day);
      if (_selectedEvents[date] != null) {
        setState(() {
          _selectedEvents[date]!.add(event);
        });
      } else if (_selectedEvents[date] == null) {
        setState(() {
          _selectedEvents[date] = [
            PostEvent(
              title: event.title,
              uid: event.uid,
              description: event.description,
              dateTimeOfEvent: DateTime(
                  event.dateTimeOfEvent.year,
                  event.dateTimeOfEvent.month,
                  event.dateTimeOfEvent.day,
                  event.dateTimeOfEvent.hour,
                  event.dateTimeOfEvent.minute),
            )
            //geohash: event.position.geohash,
            //geopoint: event.position.geopoint)
          ];
        });
      }
    }
    return _selectedEvents;
  }

  @override
  void dispose() {
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SingleChildScrollView(
        child: Column(
          children: [
            TableCalendar(
              focusedDay: selectedDay,
              firstDay: DateTime(1990),
              lastDay: DateTime(2050),
              weekendDays: const [DateTime.friday, DateTime.saturday],
              calendarFormat: format,
              onFormatChanged: (CalendarFormat _format) {
                setState(() {
                  format = _format;
                });
              },
              startingDayOfWeek: StartingDayOfWeek.sunday,
              daysOfWeekVisible: true,
              daysOfWeekStyle: const DaysOfWeekStyle(
                  weekendStyle: TextStyle(
                      color: Color.fromARGB(255, 46, 39, 39),
                      fontWeight: FontWeight.bold)),

              //Day changed
              onDaySelected: (DateTime selectDay, DateTime focusDay) {
                setState(() {
                  selectedDay = selectDay;
                  focusedDay = focusDay;
                });
              },
              selectedDayPredicate: (DateTime date) {
                return isSameDay(selectedDay, date);
              },
              availableGestures: AvailableGestures.horizontalSwipe,
              eventLoader: (day) {
                return _getEventsForDay(day);
              },

              //To style the calendar
              calendarStyle: CalendarStyle(
                  isTodayHighlighted: true,
                  markerDecoration: BoxDecoration(
                      color: Theme.of(context).primaryColorLight,
                      shape: BoxShape.circle),
                  defaultDecoration: BoxDecoration(
                    shape: BoxShape.rectangle,
                    borderRadius: BorderRadius.circular(5.0),
                  ),
                  selectedDecoration: BoxDecoration(
                    color: Theme.of(context).primaryColor,
                    shape: BoxShape.rectangle,
                    borderRadius: BorderRadius.circular(5.0),
                  ),
                  selectedTextStyle: const TextStyle(
                      color: Colors.white,
                      fontWeight: FontWeight.bold,
                      fontSize: 15.5),
                  todayDecoration: BoxDecoration(
                    color: Theme.of(context).primaryColorLight,
                    shape: BoxShape.rectangle,
                    borderRadius: BorderRadius.circular(5.0),
                  ),
                  weekendTextStyle: const TextStyle(
                    color: Color.fromARGB(255, 0, 0, 0),
                  ),
                  weekendDecoration: BoxDecoration(
                    color: Theme.of(context).primaryColorLight.withOpacity(0.2),
                    shape: BoxShape.rectangle,
                    borderRadius: BorderRadius.circular(5.0),
                  )),
              headerStyle: const HeaderStyle(
                  formatButtonVisible: true,
                  titleCentered: false,
                  formatButtonShowsNext: false,
                  formatButtonPadding:
                      EdgeInsets.symmetric(horizontal: 15, vertical: 7),
                  formatButtonTextStyle: TextStyle(
                    //color: Color.fromARGB(255, 161, 48, 48),
                    fontWeight: FontWeight.bold,
                  )),
            ),
            ..._getEventsForDay(DateTime(
                    selectedDay.year, selectedDay.month, selectedDay.day))
                .map(
              (PostEvent event) => ListTile(
                leading: const Icon(Icons.event_note_outlined),
                title: Text(event.title.toString()),
                subtitle: Text(event.description.toString()),
                trailing: const Icon(Icons.edit),
                onTap: () {
                  eventPopup(event, context);
                },
              ),
            )
          ],
        ),
      ),

When I return [PostEvent()] instead of [] inside _getEventsForDay, the markers show up for every single day on the calendar with the dummy data in the list. When I return [] instead, no markers show at all even on days that have events. I know that the if condition inside the function is being called because the list of events is displaying on the page when I tap on a date, just not the markers.

Any help would be appreciated.