add(new AjaxLink<Void>("clear") {
@Override
public void onClick(AjaxRequestTarget target) {
Reference ref = ReferenceEditorPanel.this.getModelObject();
ref.makeEmpty();
// indicate that reference was changed (might be needed if the
// model is buffered)
ReferenceEditorPanel.this.setModelObject(ref);
target.addComponent(ReferenceEditorPanel.this);
ReferenceEditorPanel.this.onUpdate(target);