How to abort a drop event??

Hello, I want to abort a drag & drop when users try to drop to an invalid target in a tree.

I am using GXT3.1, but I think the APIs are the same. How can I achieve it? Thanks.

Here is my attempt, but it does not work.

Code:

        target.addDropHandler(new DndDropHandler(){
            @Override
            public void onDrop(DndDropEvent event) {
                                // what can I do with a DragEndEvent ?
                DragEndEvent endEvent = event.getDragEndEvent();
                
                    event.getStatusProxy().setStatus(false);
                    System.out.println("drop!");
            }
        });

How to abort a drop event??