as3 Loader.load()时出现错误“Error #2025: 提供的 DisplayObject 必须是调用者的子级”的解决方案

代码<!--<br/ /><br/ />Code highlighting produced by Actipro CodeHighlighter (freeware)<br/ />http://www.CodeHighlighter.com/<br/ /><br/ />-->????????public?function?loadPhoto(picURL):void?{
????????????_loader.load(new?URLRequest(picURL));
????????????_loader.contentLoaderInfo.addEventListener(Event.COMPLETE,?onLoad);
????????????_loader.addEventListener(SecurityErrorEvent.SECURITY_ERROR,?onError);
????????}
????????
????????protected?function?onLoad(event:Event):void?{
????????????if(content.numChildren==0){
????????????????content.addChild(_loader);
????????????}
????????????
????????????_eventDispatcher.dispatchEvent(new?Event(PhotoLoader.EVENTS_LOAD));
????????}