update

Runtime/widgets/scroll_position.cs

         internal double? _pixels;
 
         public double viewportDimension {
-            get { return this._viewportDimension??0.0; }
+            get { return this._viewportDimension.Value; }
         }
 
         double? _viewportDimension;

Runtime/widgets/sliver.cs

     
         public override RenderObject createRenderObject(BuildContext context) {
             SliverMultiBoxAdaptorElement element = (SliverMultiBoxAdaptorElement)context;
-            return new RenderSliverFillViewport(childManager: element, viewportFraction: viewportFraction);
+            return new RenderSliverFillViewport(childManager: element, viewportFraction: this.viewportFraction);
-            ((RenderSliverFillViewport)renderObject).viewportFraction = viewportFraction;
+            ((RenderSliverFillViewport)renderObject).viewportFraction = this.viewportFraction;
         }
     }