fix

Runtime/widgets/page_view.cs

             return page * this.viewportDimension * this.viewportFraction;
         }
 
-        public double page => this.getPageFromPixels(this.pixels.clamp(this.minScrollExtent, this.maxScrollExtent),
-            this.viewportDimension);
+        public double page {
+            get {
+                return this.getPageFromPixels(this.pixels.clamp(this.minScrollExtent, this.maxScrollExtent),
+                    this.viewportDimension);
+            }
+        }
 
         protected override void saveScrollOffset() {
             PageStorage.of(this.context.storageContext)?.writeState(this.context.storageContext,
         public override bool applyViewportDimension(double viewportDimension) {
             double oldViewportDimensions = this.viewportDimension;
             bool result = base.applyViewportDimension(viewportDimension);
-            double oldPixels = this.pixels;
+            double? oldPixels = null;
+            if (this.hasPixles) {
+                oldPixels = this.pixels;
+            }
-                : this.getPageFromPixels(oldPixels, oldViewportDimensions);
+                : this.getPageFromPixels(oldPixels.Value, oldViewportDimensions);
             double newPixels = this.getPixelsFromPage(page);
             if (newPixels != oldPixels) {
                 this.correctPixels(newPixels);

Runtime/widgets/scroll_position.cs

 
         public override double pixels {
             get {
-                //D.assert(this._pixels != null);
+                D.assert(this._pixels != null);
+        }
+
+        public bool hasPixles {
+            get { return this._pixels != null; }
         }
 
         internal double? _pixels;