docs: public synchronized ComboBox<V> addItem(V item) {
          if(item == null) {
              throw new IllegalArgumentException("Cannot add null elements to a ComboBox");
          }
          items.add(item);
          if(selectedIndex == -1 && items.size() == 1) {
              setSelectedIndex(0);
          }
          invalidate();
          return this;
      }