fix: prevent null crash in auto-play timer when pageController.page

[laksh-scapia]

fix: prevent null crash in auto-play timer when pageController.page is null

Problem

The auto-play timer callback crashed with Null check operator used on a null value at carousel_slider.dart:144:

int nextPage = carouselState!.pageController!.page!.round() + 1;

Two compounding bugs caused this:

Bug 1 — wrong dispose() order

void dispose() {
  pageController?.dispose(); // detaches PageView → page becomes null
  super.dispose();
  clearTimer();              // timer still live during the window above
}

super.dispose() synchronously detaches the PageController from the PageView, making pageController.page return null. The timer was not cancelled until after that, leaving a window where the callback could fire against a detached controller.

Bug 2 — unsafe null-unwrap of pageController.page

pageController.page is a double? — it returns null whenever the controller is not attached to a scrollable. This can happen after super.dispose() runs, and also during the window in didUpdateWidget where a brand-new PageController is assigned to carouselState!.pageController before the widget rebuilds. The existing !mounted guard does not protect against either case.

---

Fix

1. Swap dispose() order — cancel the timer before any Flutter teardown:

void dispose() {
  clearTimer();
  pageController?.dispose();
  super.dispose();
}

2. Guard page before unwrapping — treat a null page as a bail-out signal, layered alongside the existing !mounted guard:

final double? currentPage = carouselState!.pageController!.page;
if (currentPage == null) return;
int nextPage = currentPage.round() + 1;