make Router.of nullable (#67683)

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

@@ -302,10 +302,19 @@ class Router<T> extends StatefulWidget {
   /// router to create a [ChildBackButtonDispatcher] for a nested router.
   /// Another use case may be updating the value in [routeInformationProvider]
   /// to navigate to a new route.
-  static Router<dynamic> of(BuildContext context) {
-    final _RouterScope scope = context.dependOnInheritedWidgetOfExactType<_RouterScope>()!;
-    assert(scope != null);
-    return scope.routerState.widget;
+  static Router<dynamic>? of(BuildContext context, {bool nullOk = false}) {
+    final _RouterScope? scope = context.dependOnInheritedWidgetOfExactType<_RouterScope>();
+    assert(() {
+      if (scope == null && !nullOk) {
+        throw FlutterError(
+          'Router operation requested with a context that does not include a Router.\n'
+          'The context used to retrieve the Router must be that of a widget that '
+          'is a descendant of a Router widget.'
+        );
+      }
+      return true;
+    }());
+    return scope?.routerState.widget;
   }
 
   /// Forces the [Router] to run the [callback] and reports the route

packages/flutter/test/widgets/router_test.dart

@@ -79,6 +79,28 @@ void main() {
     });
   });
 
+  testWidgets('Router.of can be null', (WidgetTester tester) async {
+    final GlobalKey key = GlobalKey();
+    await tester.pumpWidget(buildBoilerPlate(
+      Text('dummy', key: key)
+    ));
+    final BuildContext textContext = key.currentContext;
+
+    // This should not throw error.
+    Router<dynamic> router = Router.of(textContext, nullOk: true);
+    expect(router, isNull);
+
+    // Test when the nullOk is not specified.
+    bool hasFlutterError = false;
+    try {
+      router = Router.of(textContext);
+    } on FlutterError catch(e) {
+      expect(e.message.startsWith('Router'), isTrue);
+      hasFlutterError = true;
+    }
+    expect(hasFlutterError, isTrue);
+  });
+
   testWidgets('Simple router can handle pop route', (WidgetTester tester) async {
     final SimpleRouteInformationProvider provider = SimpleRouteInformationProvider();
     provider.value = const RouteInformation(