Make RouteInformationProvider value non nullable (#83301)

* Make RouteInformationProvider value non nullable

* fix more lint

packages/flutter/lib/src/widgets/router.dart

@@ -491,7 +491,7 @@ class _RouterState<T> extends State<Router<T>> with RestorationMixin {
     if (_routeInformation.value != null) {
       _processRouteInformation(_routeInformation.value!, () => widget.routerDelegate.setRestoredRoutePath);
     } else if (widget.routeInformationProvider != null) {
-      _processRouteInformation(widget.routeInformationProvider!.value!, () => widget.routerDelegate.setInitialRoutePath);
+      _processRouteInformation(widget.routeInformationProvider!.value, () => widget.routerDelegate.setInitialRoutePath);
     }
   }
 
@@ -632,7 +632,7 @@ class _RouterState<T> extends State<Router<T>> with RestorationMixin {
 
   void _handleRouteInformationProviderNotification() {
     assert(widget.routeInformationProvider!.value != null);
-    _processRouteInformation(widget.routeInformationProvider!.value!, () => widget.routerDelegate.setNewRoutePath);
+    _processRouteInformation(widget.routeInformationProvider!.value, () => widget.routerDelegate.setNewRoutePath);
   }
 
   Future<bool> _handleBackButtonDispatcherNotification() {
@@ -1300,7 +1300,7 @@ abstract class RouterDelegate<T> extends Listenable {
 ///    intent to the [Router] widget, as well as reports new route information
 ///    from the [Router] back to the engine by overriding the
 ///    [routerReportsNewRouteInformation].
-abstract class RouteInformationProvider extends ValueListenable<RouteInformation?> {
+abstract class RouteInformationProvider extends ValueListenable<RouteInformation> {
   /// A callback called when the [Router] widget detects any navigation event
   /// due to state changes.
   ///
@@ -1329,7 +1329,7 @@ class PlatformRouteInformationProvider extends RouteInformationProvider with Wid
   /// Use the [initialRouteInformation] to set the default route information for this
   /// provider.
   PlatformRouteInformationProvider({
-    RouteInformation? initialRouteInformation,
+    required RouteInformation initialRouteInformation,
   }) : _value = initialRouteInformation;
 
   @override
@@ -1343,8 +1343,8 @@ class PlatformRouteInformationProvider extends RouteInformationProvider with Wid
   }
 
   @override
-  RouteInformation? get value => _value;
-  RouteInformation? _value;
+  RouteInformation get value => _value;
+  RouteInformation _value;
 
   void _platformReportsNewRouteInformation(RouteInformation routeInformation) {
     if (_value == routeInformation)

packages/flutter/test/widgets/router_restoration_test.dart

@@ -129,9 +129,9 @@ class _TestRouterDelegate extends RouterDelegate<String> with ChangeNotifier {
 
 class _TestRouteInformationProvider extends RouteInformationProvider with ChangeNotifier {
   @override
-  RouteInformation? get value => _value;
-  RouteInformation? _value = const RouteInformation(location: '/home');
-  set value(RouteInformation? value) {
+  RouteInformation get value => _value;
+  RouteInformation _value = const RouteInformation(location: '/home');
+  set value(RouteInformation value) {
     if (value == _value) {
       return;
     }