fix a Scrollbar example crash (#127925)

Fix a scrollbar example crash.
https://api.flutter.dev/flutter/material/Scrollbar-class.html#material.Scrollbar.1

dev/bots/check_code_samples.dart

@@ -319,7 +319,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/filled_button/filled_button.0_test.dart',
   'examples/api/test/material/text_form_field/text_form_field.1_test.dart',
   'examples/api/test/material/scrollbar/scrollbar.1_test.dart',
-  'examples/api/test/material/scrollbar/scrollbar.0_test.dart',
   'examples/api/test/material/dropdown_menu/dropdown_menu.1_test.dart',
   'examples/api/test/material/radio/radio.toggleable.0_test.dart',
   'examples/api/test/material/radio/radio.0_test.dart',

examples/api/lib/material/scrollbar/scrollbar.0.dart

@@ -29,6 +29,7 @@ class ScrollbarExample extends StatelessWidget {
   Widget build(BuildContext context) {
     return Scrollbar(
       child: GridView.builder(
+        primary: true,
         itemCount: 120,
         gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 3),
         itemBuilder: (BuildContext context, int index) {

examples/api/test/material/scrollbar/scrollbar.0_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/material.dart';
+import 'package:flutter_api_samples/material/scrollbar/scrollbar.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Scrollbar.0 works well on all platforms', (WidgetTester tester) async {
+
+    await tester.pumpWidget(
+      const example.ScrollbarExampleApp(),
+    );
+
+    final Finder buttonFinder = find.byType(Scrollbar);
+    await tester.drag(buttonFinder.last, const Offset(0, 100.0));
+
+    expect(tester.takeException(), isNull);
+  }, variant: TargetPlatformVariant.all());
+}