@Override
 public void setContentView(Widget widget) {
   if (content.getElement().getChildCount() > 0) {
     for (int i = 0; i < content.getElement().getChildCount(); i++) {
       content.getElement().getChild(i).removeFromParent();
     }
   }
   content.add(widget);
 }
 @Override
 public void setNavigationView(Widget widget) {
   navigation.add(widget);
 }
 @Override
 public void setHeaderView(Widget widget) {
   header.add(widget);
 }