PopPageCallback typedef Null safety
Signature for the Navigator.onPopPage callback.
This callback must call Route.didPop on the specified route and must properly update the pages list the next time it is passed into Navigator.pages so that it no longer includes the corresponding Page. (Otherwise, the page will be interpreted as a new page to show when the Navigator.pages list is next updated.)
typedef PopPageCallback = bool Function(Route<dynamic> route, dynamic result);