code: public synchronized ComboBox<V> setItem(int index, V item) {
          if(item == null) {
              throw new IllegalArgumentException("Cannot add null elements to a ComboBox");
          }
          items.set(index, item);
          invalidate();
          return this;
      }
docs: Updates the combo box so the item at the specified index is swapped out with the supplied value in the
  {@code item} parameter
  @param index Index of the item to swap out
  @param item Item to replace with
  @return Itself