code: protected void onRootChoiceUpdate(final AjaxRequestTarget target) {
  		childTextField.setModelObject(getModelObject().getSelectedChildOption());
  		target.add(DropdownAutocompleteTextFieldPanel.this.childTextField);
  	}
docs: Callback method that can be overwritten to provide an additional action
  when root choice has updated.
  
  @param target
  the current request handler