onChanged property
Called when the user selects an item.
If the onChanged callback is null or the list of DropdownButton2.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 DropdownButton2.disabledHint widget if it is non-null.
If DropdownButton2.disabledHint is also null but DropdownButton2.hint is
non-null, DropdownButton2.hint will instead be displayed.
Implementation
final ValueChanged<T?>? onChanged;