Add more to error message of RestorationScope.of (#126444)

Fixes https://github.com/flutter/flutter/issues/126443

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

@@ -106,15 +106,26 @@ class RestorationScope extends StatefulWidget {
     final RestorationBucket? bucket = maybeOf(context);
     assert(() {
       if (bucket == null) {
-        throw FlutterError(
-          'RestorationScope.of() was called with a context that does not contain a '
-          'RestorationScope widget.\n'
-          'No RestorationScope widget ancestor could be found starting from the '
-          'context that was passed to RestorationScope.of(). This can happen '
-          'because you are using a widget that looks for a RestorationScope '
-          'ancestor, but no such ancestor exists.\n'
-          'The context used was:\n'
-          '  $context',
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary(
+            'RestorationScope.of() was called with a context that does not '
+            'contain a RestorationScope widget. '
+          ),
+          ErrorDescription(
+            'No RestorationScope widget ancestor could be found starting from '
+            'the context that was passed to RestorationScope.of(). This can '
+            'happen because you are using a widget that looks for a '
+            'RestorationScope ancestor, but no such ancestor exists.\n'
+            'The context used was:\n'
+            '  $context'
+          ),
+          ErrorHint(
+            'State restoration must be enabled for a RestorationScope to exist. '
+            'This can be done by passing a restorationScopeId to MaterialApp, '
+            'CupertinoApp, or WidgetsApp at the root of the widget tree or by '
+            'wrapping the widget tree in a RootRestorationScope.'
+          ),
+        ],
         );
       }
       return true;

packages/flutter/test/widgets/restoration_scope_test.dart

@@ -51,6 +51,52 @@ void main() {
   });
 
   group('RestorationScope', () {
+    testWidgets('asserts when none is found', (WidgetTester tester) async {
+      late BuildContext capturedContext;
+      await tester.pumpWidget(WidgetsApp(
+        color: const Color(0xD0FF0000),
+        builder: (_, __) {
+          return RestorationScope(
+            restorationId: 'test',
+            child: Builder(
+              builder: (BuildContext context) {
+                capturedContext = context;
+                return Container();
+              }
+            )
+          );
+        },
+      ));
+      expect(
+        () {
+          RestorationScope.of(capturedContext);
+        },
+        throwsA(isA<FlutterError>().having(
+          (FlutterError error) => error.message,
+          'message',
+          contains('State restoration must be enabled for a RestorationScope'),
+        )),
+      );
+
+      await tester.pumpWidget(WidgetsApp(
+        restorationScopeId: 'test scope',
+        color: const Color(0xD0FF0000),
+        builder: (_, __) {
+          return RestorationScope(
+            restorationId: 'test',
+            child: Builder(
+              builder: (BuildContext context) {
+                capturedContext = context;
+                return Container();
+              }
+            )
+          );
+        },
+      ));
+      final UnmanagedRestorationScope scope = tester.widget(find.byType(UnmanagedRestorationScope).last);
+      expect(RestorationScope.of(capturedContext), scope.bucket);
+    });
+
     testWidgets('makes bucket available to descendants', (WidgetTester tester) async {
       const String id = 'hello world 1234';
       final MockRestorationManager manager = MockRestorationManager();