The list of items the user can select.
If the onChanged callback is null or the list of items is null then the dropdown button will be disabled, i.e. its arrow will be displayed in grey and it will not respond to input. A disabled button will display the disabledHint widget if it is non-null. If disabledHint is also null but hint is non-null, hint will instead be displayed.
final List<DropdownMenuItem<T>> items