Can not replace the field in VerticalFieldManager

I am using following code

UiApplication.getUiApplication().invokeLater(new Runnable(){

     public void run(){
         // your code here
         XmlDataManager xml=new XmlDataManager();
         BitmapField f=xml.getImageFromUrl("http://wiztech.pk/ali/event/"+catg);
         // main1.replace(loading, f);
         // t开发者_如何转开发his.setTitle(f);
     }
});

To replace the field in VerticalFieldManager, but it throws exception, can any one help please what us wrong with this?

The only thing i can think of is either f is null. which means getImageFromUrl is not returning a BitmapField. Or loading has already been added to another field manager