mainPanel.add(successPanel);

mainPanel.add(errorPanel);

public static void removeWidget(Widget w) {
    if (w.getParent() == null) {
      RootPanel.get().add(w);
      if (RootPanel.isInDetachList(w)) {
        RootPanel.detachNow(w);
      } else {
        try {
          // onUnload() gets called *before* everything else (the opposite of
          // onLoad()).
          w.onUnload();
          AttachEvent.fire(w, false);
        } catch (Exception e) {
          // ??? throws "Should only call onDetach when the widget is attached to the browser's document"
        } finally {
          // Put this in a finally, just in case onUnload throws an exception.
          try {
            w.doDetachChildren();
          } catch (Exception e) {
            // ???
          } finally {
            // Put this in a finally, in case doDetachChildren throws an exception.
            DOM.setEventListener(w.getElement(), null);
            //w.attached = false; // ??
          }
        }
      }
    } else if (w instanceof HasWidgets) {
      ((HasWidgets) w).remove(w);
    } else if (w != null) {
      throw new IllegalStateException("This widget's parent does not implement HasWidgets");
    }

  }