fix shopping cart page error

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/shrine/shopping_cart.cs

 using Unity.UIWidgets.rendering;
 using Unity.UIWidgets.ui;
 using Unity.UIWidgets.widgets;
+using UnityEngine;
 using Image = Unity.UIWidgets.widgets.Image;
 using TextStyle = Unity.UIWidgets.painting.TextStyle;
 
   List<Widget> _createShoppingCartRows(AppStateModel model)
   {
     List<Widget> widgets = new List<Widget>();
-    for (int id = 0; id < model.productsInCart.Count; id++)
+    foreach (var product in model.productsInCart.Keys)
-      widgets.Add(new ShoppingCartRow(
-        product: model.getProductById(id),
-        quantity: model.productsInCart[id],
-        onPressed: ()=> {
-          model.removeItemFromCart(id);
-        }
-      ));
+      int id = product;
+        widgets.Add(new ShoppingCartRow(
+          product: model.getProductById(id),
+          quantity: model.productsInCart[id],
+          onPressed: ()=> {
+            model.removeItemFromCart(id);
+          }
+        ));
     }
 
     return widgets;
 public class ShoppingCartRow : StatelessWidget {
   public ShoppingCartRow(
     Product product = null,
-    int? quantity = null,
+    int quantity = 1,
-    this.quantity = quantity?? 0;
+    this.quantity = quantity;
     this.onPressed = onPressed;
   }
 
                   new Row(
                     crossAxisAlignment: CrossAxisAlignment.start,
                     children: new List<Widget>{
-                      Image.asset(
+                      Image.file(
-                        package: product.assetPackage,
+                        //package: product.assetPackage,
                         fit: BoxFit.cover,
                         width: 75.0f,
                         height: 75.0f
                                 new Expanded(
                                   child: new Text($"Quantity: {quantity}")
                                 ),
-                                new Text($"x  $ {product.price : D} ")
+                                new Text($"x  $ {product.price :F} ")
                               }
                             ),
                             new Text(

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/shrine/supplemental/product_card.cs

 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.rendering;
 using Unity.UIWidgets.widgets;
+using UnityEngine;
 
 namespace UIWidgetsGallery.demo.shrine.supplemental
 {
                                             ),
                                             new SizedBox(height: 4.0f),
                                             new Text(
-                                                $" $ { product.price:D}",
+                                                $" $ { product.price:F}",
                                                 style: theme.textTheme.caption
                                             ),
                                         }

com.unity.uiwidgets/Runtime/widgets/scroll_position.cs

         internal readonly object _coordinator;
 
         public float minScrollExtent {
-            get { return _minScrollExtent.Value; }
+            get { return _minScrollExtent; }
-        float? _minScrollExtent;
+        float _minScrollExtent;
-            get { return _maxScrollExtent.Value; }
-        }
-
-        float? _maxScrollExtent;
-
-        public bool hasMinScrollExtent {
-            get { return _minScrollExtent != null; }
+            get { return _maxScrollExtent; }
-        public bool hasMaxScrollExtent {
-            get { return _maxScrollExtent != null; }
-        }
+        float _maxScrollExtent;
 
         public override float pixels {
             get {
         internal float? _pixels;
 
         public float viewportDimension {
-            get { return _viewportDimension.Value; }
+            get { return _viewportDimension; }
-        float? _viewportDimension;
+        float _viewportDimension;
 
         public bool haveDimensions {
             get { return _haveDimensions; }