调试Flex框架错误的技术

我时不时会遇到Flex框架抛出的异常。通常来自呈现itemrender等的ListBase代码。我正在寻找一些技术来找出哪一个ListBase实例甚至抛出了错误。

当调用堆栈的顶部起源于callLaterDispatcher()时,困难就出现了

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at mx.controls.listClasses::ListBase/addToFreeItemRenderers()
    at mx.controls.listClasses::ListBase/reduceRows()
    at mx.controls.listClasses::ListBase/updateDisplayList()
    at mx.controls.listClasses::TileBase/updateDisplayList()
    at mx.controls.listClasses::ListBase/validateDisplayList()
    at mx.managers::LayoutManager/validateDisplayList()
    at mx.managers::LayoutManager/doPhasedInstantiation()
    at Function/http://adobe.com/AS3/2006/builtin::apply()
    at mx.core::UIComponent/callLaterDispatcher2()
    at mx.core::UIComponent/callLaterDispatcher()

protected function addToFreeItemRenderers(item:IListItemRenderer):void
{
    // The following item is NULL when the exception is being thrown...
    if (item == null) return;

    DisplayObject(item).visible = false;

    var factory:IFactory = factoryMap[item];    
...

我该怎么弄清楚我需要弄清楚什么?我以前也解决过类似的问题,但我不得不使用魔法和巫毒,猜测并在代码中加入callLater调用来修复。