Bug Fix: Bug when setting initialDate to December in ScrollWheelDatePicker

[vietdq2701]

Fixes #9

Problem

When setting initialDate to a date in December, the ScrollWheelDatePicker throws an error due to state changes being triggered during the widget's build phase. This issue occurs because the DateController calls notifyListeners directly during initialization.

Solution

• Added a WidgetsBinding.instance.addPostFrameCallback to delay notifyListeners until after the current frame is built. This prevents the error caused by state changes during the build phase.
• Updated _dayController initialization to properly handle the month value, ensuring December (month 12) is processed correctly.

Impact

• Fixes the crash when initialDate is set to December.
• Improves stability by ensuring notifyListeners is only called after the build phase is complete.
• No breaking changes for existing implementations.

Code Changes

Before (causing the error):

_dayController = _DayController(
  selectedIndex: initialDate != null ? initialDate.day - 1 : null,
  numberOfDays: _getNumberOfDays(
    year: initialDate?.year ?? DateTime.now().year,
    month: initialDate?.month ?? DateTime.now().month,
  ),
);

  void _updateNumberOfDays() {
    final int numberOfDays = _getNumberOfDays(year: _yearController.selectedIndex, month: _monthController.selectedIndex);

    final int selectedIndex = _dayController.selectedIndex >= numberOfDays ? numberOfDays - 1 : _dayController.selectedIndex;

    _dayController = _dayController.copyWith(selectedIndex: selectedIndex, numberOfDays: numberOfDays);

    notifyListeners();
  }

After fix

_dayController = _DayController(
  selectedIndex: initialDate != null ? initialDate.day - 1 : null,
  numberOfDays: _getNumberOfDays(
    year: initialDate?.year ?? DateTime.now().year,
    month: (initialDate != null) ? initialDate.month - 1 : DateTime.now().month - 1,
  ),
);


void _updateNumberOfDays() {
    final int numberOfDays = _getNumberOfDays(year: _yearController.selectedIndex, month: _monthController.selectedIndex);

    final int selectedIndex = _dayController.selectedIndex >= numberOfDays ? numberOfDays - 1 : _dayController.selectedIndex;

    _dayController = _dayController.copyWith(selectedIndex: selectedIndex, numberOfDays: numberOfDays);

    WidgetsBinding.instance.addPostFrameCallback((_) {
      notifyListeners();
    });
  }