Hi there,
It's very easy to replicate error:

1. Define

@override
String getRecurrenceRule(int index) { return 'FREQ=WEEKLY;INTERVAL=1;' }

2. It throws an error:

E/flutter (10612): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: Invalid argument(s)
E/flutter (10612): #0 List.[] (dart:core-patch/growable_array.dart:146:60)
E/flutter (10612): #1 _getRecurrenceDateTimeCollection (package:syncfusion_flutter_calendar/src/calendar/appointment_engine/recurrence_helper.dart:124:47)
E/flutter (10612): #2 _getRecurrenceAppointments (package:syncfusion_flutter_calendar/src/calendar/appointment_engine/appointment_helper.dart:1416:20)
E/flutter (10612): #3 _getVisibleAppointments (package:syncfusion_flutter_calendar/src/calendar/appointment_engine/appointment_helper.dart:1240:5)
E/flutter (10612): #4 _SfCalendarState._updateVisibleAppointments (package:syncfusion_flutter_calendar/src/calendar/sfCalendar.dart:1079:36)
E/flutter (10612): #5 _SfCalendarState._getAppointment (package:syncfusion_flutter_calendar/src/calendar/sfCalendar.dart:1071:5)
E/flutter (10612): #6 _SfCalendarState.didUpdateWidget (package:syncfusion_flutter_calendar/src/calendar/sfCalendar.dart:1446:7)
E/flutter (10612): #7 StatefulElement.update (package:flutter/src/widgets/framework.dart:4690:58)
E/flutter (10612): #8 Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
E/flutter (10612): #9 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:5522:32)
E/flutter (10612): #10 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5957:17)
E/flutter (10612): #11 Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
E/flutter (10612): #12 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4527:16)
E/flutter (10612): #13 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
E/flutter (10612): #14 Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
E/flutter (10612): #15 StatefulElement.update (package:flutter/src/widgets/framework.dart:4707:5)
E/flutter (10612): #16 Element.updateChild (package:flutter/src/widgets/framework.dart:3201:15)
E/flutter (10612): #17 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4527:16)
E/flutter (10612): #18 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
E/flutter (10612): #19 Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
E/flutter (10612): #20 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2627:33)
E/flutter (10612): #21 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:883:20)
E/flutter (10612): #22 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:284:5)
E/flutter (10612): #23 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1113:15)
E/flutter (10612): #24 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1052:9)
E/flutter (10612): #25 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:968:5)
E/flutter (10612): #26 _rootRun (dart:async/zone.dart:1184:13)
E/flutter (10612): #27 _CustomZone.run (dart:async/zone.dart:1077:19)
E/flutter (10612): #28 _CustomZone.runGuarded (dart:async/zone.dart:979:7)
E/flutter (10612): #29 _invoke (dart:ui/hooks.dart:261:10)
E/flutter (10612): #30 _drawFrame (dart:ui/hooks.dart:219:3)