CalendarDataSource should supply type information...
Example in `calendar_data_source.dart` -
Example showing data source with custom `Appointment` type:
Without type information, you end up with errors:
EDIT: another place where type information should be able to be specified is on `CalendarTapDetails` and the callback.
In general, type info should be able to be specified anywhere where we access a custom appointment type through the syncfusion API.