TypeError: Error #1034: 强制转换类型失败:无法将 mx.controls::DataGrid@9a7c0a1 转换为 spark.core.IViewport。

TypeError: Error #1034: 强制转换类型失败:无法将 mx.controls::DataGrid@9aa90a1 转换为 spark.core.IViewport。
at mx.binding::Binding/defaultDestFunc()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\binding\Binding.as:270]
at Function/http://adobe.com/AS3/2006/builtin::call()
at mx.binding::Binding/innerExecute()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\binding\Binding.as:475]
at Function/http://adobe.com/AS3/2006/builtin::apply()
at mx.binding::Binding/wrapFunctionCall()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\binding\Binding.as:385]
at mx.binding::Binding/execute()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\binding\Binding.as:321]
at mx.binding::BindingManager$/executeBindings()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\binding\BindingManager.as:153]
at mx.core::UIComponent/executeBindings()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\UIComponent.as:10691]
at mx.core::Container/createComponentFromDescriptor()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\Container.as:4373]
at mx.core::Container/createComponentsFromDescriptors()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\Container.as:4160]
at mx.core::Container/createChildren()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\Container.as:3187]
at mx.core::UIComponent/initialize()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7250]
at mx.core::Container/initialize()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\Container.as:3129]
at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7142]
at mx.core::UIComponent/addChildAt()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\UIComponent.as:6848]
at spark.components::Group/addDisplayObjectToDisplayList()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\Group.as:1825]
at spark.components::Group/http://www.adobe.com/2006/flex/mx/internal::elementAdded()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\Group.as:1416]
at spark.components::Group/setMXMLContent()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\Group.as:512]
at spark.components::Group/set mxmlContent()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\Group.as:452]
at spark.components::SkinnableContainer/set mxmlContent()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\SkinnableContainer.as:604]
at spark.components::SkinnableContainer/createDeferredContent()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\SkinnableContainer.as:986]
at spark.components::SkinnableContainer/createContentIfNeeded()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\SkinnableContainer.as:1014]
at spark.components::SkinnableContainer/createChildren()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\SkinnableContainer.as:827]
at mx.core::UIComponent/initialize()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7250]
at spark.components::Application/initialize()[E:\dev\4.0.0\frameworks\projects\spark\src\spark\components\Application.as:916]
at Table/initialize()
at mx.managers.systemClasses::ChildManager/childAdded()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\systemClasses\ChildManager.as:189]
at mx.managers.systemClasses::ChildManager/initializeTopLevelWindow()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\systemClasses\ChildManager.as:341]
at mx.managers::SystemManager/initializeTopLevelWindow()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:2810]
at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::kickOff()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:2637]
at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::preloader_completeHandler()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:2539]
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.preloaders::Preloader/timerHandler()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\preloaders\Preloader.as:515]
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()

/**
* 刷新按钮函数
*/
protected function refresh_clickHandler(event:MouseEvent):void
{
var hx:Number = hsb.x;
var hy:Number = hsb.y;

//var dx:Number = dataGrid.contentMouseX;
//var dy:Number = dataGrid.contentMouseY;
//Alert.show("hx："+hx+"\n"+"hy: "+hy + "\n" + "dx: " + dx + "\n" + "dy: " + dy);
Alert.show("hx："+hx+"\n"+"hy: "+hy);
}

<s:HScrollBar id="hsb" width="100%" viewport="{dataGrid}"/>