https://main-api.flutter.dev/flutter/material/DropdownMenu/onSelected.html

The callback is called when a selection is made.

Defaults to null. If null, only the text field is updated.

Upon reading the field / class docs, it does not tell me what null represents in T? and when I would get null in the callback.

Also since null seems to have a special meaning to the DropdownMenu implementation, the class needs an additional type constraint: T extends Object so the user can't assign a different meaning to null.