The route generator callback used when the app is navigated to a
If this returns null when building the routes to handle the specified
initialRoute, then all the routes are discarded and
Navigator.defaultRouteName is used instead (
/). See initialRoute.
During normal app operation, the onGenerateRoute callback will only be
applied to route names pushed by the application, and so should never
This is used if routes does not contain the requested route.
The Navigator is only built if routes are provided (either via home,
routes, onGenerateRoute, or onUnknownRoute); if they are not,
builder must not be null.
If this property is not set, either the routes or home properties must
be set, and the pageRouteBuilder must also be set so that the
default handler will know what routes and PageRoutes to build.
final RouteFactory? onGenerateRoute;