Merge pull request #133 from UnityTech/textselection

fix assertion triggered by disposing a pageView before its initializa…

Runtime/widgets/binding.cs

                 return;
             }
             
+            //The former widget tree must be layout first before its destruction
+            this.drawFrame();
             this.attachRootWidget(null);
             this.buildOwner.buildScope(this._renderViewElement);
             this.buildOwner.finalizeTree();