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/desktop/shop.dart

@@ -32,13 +32,13 @@ class Shop extends StatefulWidget {
 }
 
 class ShopState extends State<Shop> {
-  Business _business;
+  late Business _business;
 
   PanelController panelController = PanelController();
-  SlidingUpPanel _slidUpShoppingCart;
+  late SlidingUpPanel _slidUpShoppingCart;
   GlobalKey endKey = GlobalKey();
 
-  List<CategoryProducts> _categoryProducts;
+  late List<CategoryProducts> _categoryProducts;
   bool displayProductByCategoryClick = false;
   String displayProductByCategoryClickIndicator = '';
   int categoryId = 0;
@@ -261,7 +261,7 @@ class ShopState extends State<Shop> {
             if (moreCategoryProducts.isEmpty) {
               _productCurrentPage = 0;
             } else {
-              if (moreCategoryProducts[0].products.length < Constants.ORDERS_PER_PAGE) {
+              if (moreCategoryProducts[0].products!.length < Constants.ORDERS_PER_PAGE) {
                 _productCurrentPage = 0;
               }
             }