phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/widgets/general/carousel.dart

@@ -6,8 +6,8 @@ import 'style.dart';
 class Carousel extends StatefulWidget {
   Carousel({
     double height = 200.0,
-    List<Widget> pages,
-    bool autoPlay,
+    List<Widget>? pages,
+    bool? autoPlay,
     Duration duration = const Duration(seconds: 2),
     Duration animationDuration = const Duration(milliseconds: 1000),
   })
@@ -30,7 +30,7 @@ class Carousel extends StatefulWidget {
 class CarouselState extends State<Carousel> {
   final _pageController = new PageController();
 
-  Timer _timer;
+  late Timer _timer;
   int _currentPage = 0;
   bool reverse = false;
   GlobalKey<IndicatorState> _indicatorStateKey = new GlobalKey();
@@ -80,7 +80,7 @@ class CarouselState extends State<Carousel> {
             children: widget.pages,
             onPageChanged: (index) {
               _currentPage = index;
-              _indicatorStateKey.currentState.changeIndex(index);
+              _indicatorStateKey.currentState!.changeIndex(index);
             },
           ),
         ),
@@ -102,7 +102,7 @@ class CarouselState extends State<Carousel> {
 }
 
 class Indicator extends StatefulWidget {
-  Indicator({Key? key, int count})
+  Indicator({Key? key, int? count})
       : count = count,
         super(key: key);