code: public synchronized T addItem(V item) {
          if(item == null) {
              return self();
          }
  
          items.add(item);
          if(selectedIndex == -1) {
              selectedIndex = 0;
          }
          invalidate();
          return self();
      }
docs: Adds one more item to the list box, at the end.
  @param item Item to add to the list box
  @return Itself