code: @Override
  	public void addGroup(RibbonGroup ribbonGroup) {
  		if (null == ribbonGroup) {
  			throw new IllegalArgumentException("Cannot add RibbonGroup with null value.");
  		}
  		groups.add(ribbonGroup);
  		ribbonGroup.setShowTitle(showGroupTitles);
  		ribbonGroup.asWidget().setStyleName(getStyleName() + "Group");
  		addMember(ribbonGroup.asWidget());
  	}
docs: ------------------------------------------------------------------------