Properly initialize RestorationManager in the TestBinding (#70398)

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

@@ -301,7 +301,9 @@ class _RootRestorationScopeState extends State<RootRestorationScope> {
 
   @override
   void dispose() {
-    ServicesBinding.instance!.restorationManager.removeListener(_replaceRootBucket);
+    if (_rootBucketValid) {
+      ServicesBinding.instance!.restorationManager.removeListener(_replaceRootBucket);
+    }
     super.dispose();
   }
 

packages/flutter_test/lib/src/binding.dart

@@ -189,13 +189,16 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   final TestWindow _window;
 
   @override
-  TestRestorationManager get restorationManager => _restorationManager;
-  late TestRestorationManager _restorationManager;
+  TestRestorationManager get restorationManager {
+    _restorationManager ??= createRestorationManager();
+    return _restorationManager!;
+  }
+  TestRestorationManager? _restorationManager;
 
   /// Called by the test framework at the beginning of a widget test to
   /// prepare the binding for the next test.
   void reset() {
-    _restorationManager = createRestorationManager();
+    _restorationManager = null;
     resetGestureBinding();
   }
 

packages/flutter_test/test/bindings_restoration_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('Can access restoration manager without crashing', () {
+    final AutomatedTestWidgetsFlutterBinding binding = AutomatedTestWidgetsFlutterBinding();
+    expect(binding.restorationManager, isA<RestorationManager>());
+  });
+}