builder property Null safety
A builder for inserting widgets above the Navigator or - when the WidgetsApp.router constructor is used - above the Router but below the other widgets created by the WidgetsApp widget, or for replacing the Navigator/Router entirely.
For example, from the BuildContext passed to this method, the Directionality, Localizations, DefaultTextStyle, MediaQuery, etc, are all available. They can also be overridden in a way that impacts all the routes in the Navigator or Router.
This is rarely useful, but can be used in applications that wish to override those defaults, e.g. to force the application into right-to-left mode despite being in English, or to override the MediaQuery metrics (e.g. to leave a gap for advertisements shown by a plugin from OEM code).
If no routes are provided to the regular WidgetsApp constructor using
home, routes, onGenerateRoute, or onUnknownRoute, the
be null, and it is the responsibility of the builder to provide the
application's routing machinery.
If routes are provided to the regular WidgetsApp constructor using one
or more of those properties or if the WidgetsApp.router constructor is
child is not null, and the returned value should include the
child in the widget subtree; if it does not, then the application will
have no Navigator or Router and the routing related properties (i.e.
navigatorKey, home, routes, onGenerateRoute, onUnknownRoute,
initialRoute, navigatorObservers, routeInformationProvider,
backButtonDispatcher, routerDelegate, and routeInformationParser)
If builder is null, it is as if a builder was specified that returned
child directly. If it is null, routes must be provided using one of
the other properties listed above.
Unless a Navigator is provided, either implicitly from builder being
null, or by a builder including its
child argument, or by a builder
explicitly providing a Navigator of its own, or by the routerDelegate
building one, widgets and APIs such as Hero, Navigator.push and
Navigator.pop, will not function.
final TransitionBuilder? builder;