code: private String getValueForId(String id) {
          for (int k = 0; k < fieldset.getWidgetCount(); k++) {
              Widget widget = fieldset.getWidget(k);
              if (id.equals(widget.getElement().getAttribute("id")))
                  return widget.getElement().getAttribute("value");
          }
          return null;
      }
docs: Returns the value of the button that has the given id
  
  @return the value of the button with the given id