public void addItem(final T item) {
        if (item != null) {
            items.add(wrap(item));
        }
    }
This function will be used to optionally provide a rendered form of the item instead of plain text.
If not set then it will fall back on displayValueFunction, {@link HasDisplayValue} or
{@link Object#toString()}.
stroom-core-client-widget/src/main/java/stroom/item/client/SimpleSelectionListModel.java