Don't visit element children of the childless (#10558)

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

@@ -140,7 +140,8 @@ class _SliverPersistentHeaderElement extends RenderObjectElement {
 
   @override
   void visitChildren(ElementVisitor visitor) {
-    visitor(child);
+    if (child != null)
+      visitor(child);
   }
 }
 

packages/flutter/test/material/tabbed_scrollview_warp_test.dart

@@ -0,0 +1,82 @@
+// Copyright 2017 The Chromium 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_test/flutter_test.dart';
+import 'package:flutter/material.dart';
+
+// This is a regression test for https://github.com/flutter/flutter/issues/10549
+// which was failing because _SliverPersistentHeaderElement.visitChildren()
+// didn't check child != null before visiting its child.
+
+class MySliverPersistentHeaderDelegate extends SliverPersistentHeaderDelegate {
+  @override
+  double get minExtent => 50.0;
+
+  @override
+  double get maxExtent => 150.0;
+
+  @override
+  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) => const Placeholder(color: Colors.teal);
+
+  @override
+  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate) => false;
+}
+
+class MyHomePage extends StatefulWidget {
+  @override
+  _MyHomePageState createState() => new _MyHomePageState();
+}
+
+class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
+  static const int tabCount = 3;
+  TabController tabController;
+
+  @override
+  void initState() {
+    super.initState();
+    tabController = new TabController(initialIndex: 0, length: tabCount, vsync: this);
+  }
+
+  @override
+  void dispose() {
+    tabController.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Scaffold(
+      appBar: new AppBar(
+        bottom: new TabBar(
+          controller: tabController,
+          tabs: new List<Widget>.generate(tabCount, (int index) => new Tab(text: 'Tab $index')).toList(),
+        ),
+      ),
+      body: new TabBarView(
+        controller: tabController,
+        children: new List<Widget>.generate(tabCount, (int index) {
+          return new CustomScrollView(
+            // The bug only occurs when this key is included
+            key: new ValueKey<String>('Page $index'),
+            slivers: <Widget>[
+              new SliverPersistentHeader(
+                delegate: new MySliverPersistentHeaderDelegate(),
+              ),
+            ],
+          );
+        }).toList(),
+      ),
+    );
+  }
+}
+
+void main() {
+  testWidgets('Tabbed CustomScrollViews, warp from tab 1 to 3', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(home: new MyHomePage()));
+
+    // should not crash.
+    await tester.tap(find.text('Tab 2'));
+    await tester.pumpAndSettle();
+  });
+}