Java 无法在gxt中的flowlayoutcontainer中拖放文本框

我想从垂直布局容器中拖动一个文本框，然后将其放入流布局容器中。放置时，它应显示为一个富文本区域。请在下面查找代码。我试着调试代码，但它只在拖动中输入。调试未进入droptarget内部。你能帮帮我吗

final TextButton textButton = new TextButton();
    textButton.setText("Text Box");
    DragSource source = new DragSource(textButton) {
        @Override
        protected void onDragStart(DndDragStartEvent event) {
          super.onDragStart(event);
        }
      };

      DropTarget  dropTarget = new DropTarget(flowLayoutContainer);
      dropTarget.setOperation(Operation.COPY);
      dropTarget.addDropHandler(new DndDropHandler() {
            @Override
            public void onDrop(DndDropEvent event) {
                final RichTextArea textBox1 = new RichTextArea();
                flowLayoutContainer.add(textBox1);
            }
        });