fix assertion triggered by disposing a pageView before its initialization, which is done on its first PerformLayout call

Author: zhuxingwei

Runtime/widgets/binding.cs

@@ -143,6 +143,8 @@ public void detachRootWidget() {
                 return;
             }
 
+            //draw one frame to ensure that the former widget tree has been layout for at least once
+            this.drawFrame();
             this.attachRootWidget(null);
             this.buildOwner.buildScope(this._renderViewElement);
             this.buildOwner.finalizeTree();