fix an issue for page view.

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

             this.dragStartBehavior = dragStartBehavior;
             this.controller = controller ?? PageViewUtils._defaultPageController;
             this.allowImplicitScrolling = allowImplicitScrolling;
-            childrenDelegate = new SliverChildListDelegate(children);
+            childrenDelegate = new SliverChildListDelegate(children ?? new List<Widget>());
         }
         
         public static PageView builder(