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