code: protected void addMainWidget(Widget widget) {
  
          assert m_mainWidget == null;
          assert m_listItemWidget == null;
          if (widget instanceof CmsListItemWidget) {
              m_listItemWidget = (CmsListItemWidget)widget;
          }
          m_mainWidget = widget;
      }
docs: Adds the main widget to the list item.<p>
  
  In most cases, the widget will be a list item widget. If this is the case, then further calls to {@link CmsListItem#getListItemWidget()} will
  return the widget which was passed as a parameter to this method. Otherwise, the method will return null.<p>
  
  @param widget the main content widget