Error #1009 when scroll the verticalscrollbar in textarea

Hi All,

     I create a simple use flash builder 4,main code like

             <fx:String id="htmlText">

                    <![CDATA[

                         <p><img src="imagepath"/></p>

                         <p><img src="imagepath"/></p>

                    ]]>

             </fx:String>

             <s:TextArea width="580" height="580" textFlow="{TextConverter.importToFlow(htmlText, TextConverter.TEXT_FIELD_HTML_FORMAT)}"/>

    It's work good when image size small than the textarea.If image size overflow,when I scroll the verticalscrollbar there comes out:

    TypeError: Error #1009: Cannot access a property or method of a null object reference.
        at flashx.textLayout.container::ContainerController/getScrollDelta()
        at  flashx.textLayout.container::TextContainerManager/getScrollDelta()
        at  spark.components::RichEditableText/getVerticalScrollPositionDelta()
        at  spark.components::VScrollBar/changeValueByStep()
        at  spark.components.supportClasses::ScrollBarBase/button_buttonDownHandler()
        at flash.events::EventDispatcher/dispatchEventFunction()
        at flash.events::EventDispatcher/dispatchEvent()
        at mx.core::UIComponent/dispatchEvent()
        at  spark.components.supportClasses::ButtonBase/commitProperties()
        at mx.core::UIComponent/validateProperties()
        at mx.managers:：LayoutManager/validateProperties()
        at mx.managers:：LayoutManager/doPhasedInstantiation()
        at mx.managers:：LayoutManager/doPhasedInstantiationCallback()

        please help!

        thanks