From 57e81894b2ac61b20412b5b9afcde632144cfc34 Mon Sep 17 00:00:00 2001
From: Greg Spencer <gspencergoog@users.noreply.github.com>
Date: Thu, 23 Jan 2020 20:23:02 -0800
Subject: [PATCH] Guard against a null navigator on popping a route. (#49329)

---
 .../flutter/lib/src/widgets/navigator.dart    |  2 +-
 .../flutter/test/widgets/navigator_test.dart  | 48 +++++++++++++++++++
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/packages/flutter/lib/src/widgets/navigator.dart b/packages/flutter/lib/src/widgets/navigator.dart
index a8d0f9bf23..e624cc5bc9 100644
--- a/packages/flutter/lib/src/widgets/navigator.dart
+++ b/packages/flutter/lib/src/widgets/navigator.dart
@@ -136,7 +136,7 @@ abstract class Route<T> {
   @mustCallSuper
   TickerFuture didPush() {
     return TickerFuture.complete()..then<void>((void _) {
-      navigator.focusScopeNode.requestFocus();
+      navigator?.focusScopeNode?.requestFocus();
     });
   }
 
diff --git a/packages/flutter/test/widgets/navigator_test.dart b/packages/flutter/test/widgets/navigator_test.dart
index 5522c1179d..9f87b52e58 100644
--- a/packages/flutter/test/widgets/navigator_test.dart
+++ b/packages/flutter/test/widgets/navigator_test.dart
@@ -1036,6 +1036,54 @@ void main() {
     expect(find.byKey(keyAB), findsNothing);
   });
 
+  testWidgets("Popping immediately after pushing doesn't crash", (WidgetTester tester) async {
+    // Added this test to protect against regression of https://github.com/flutter/flutter/issues/45539
+    final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
+      '/' : (BuildContext context) => OnTapPage(id: '/', onTap: () {
+        Navigator.pushNamed(context, '/A');
+        Navigator.of(context).pop();
+      }),
+      '/A': (BuildContext context) => OnTapPage(id: 'A', onTap: () { Navigator.pop(context); }),
+    };
+    bool isPushed = false;
+    bool isPopped = false;
+    final TestObserver observer = TestObserver()
+      ..onPushed = (Route<dynamic> route, Route<dynamic> previousRoute) {
+        // Pushes the initial route.
+        expect(route is PageRoute && route.settings.name == '/', isTrue);
+        expect(previousRoute, isNull);
+        isPushed = true;
+      }
+      ..onPopped = (Route<dynamic> route, Route<dynamic> previousRoute) {
+        isPopped = true;
+      };
+
+    await tester.pumpWidget(MaterialApp(
+      routes: routes,
+      navigatorObservers: <NavigatorObserver>[observer],
+    ));
+    expect(find.text('/'), findsOneWidget);
+    expect(find.text('A'), findsNothing);
+    expect(isPushed, isTrue);
+    expect(isPopped, isFalse);
+
+    isPushed = false;
+    isPopped = false;
+    observer.onPushed = (Route<dynamic> route, Route<dynamic> previousRoute) {
+      expect(route is PageRoute && route.settings.name == '/A', isTrue);
+      expect(previousRoute is PageRoute && previousRoute.settings.name == '/', isTrue);
+      isPushed = true;
+    };
+
+    await tester.tap(find.text('/'));
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1));
+    expect(find.text('/'), findsOneWidget);
+    expect(find.text('A'), findsNothing);
+    expect(isPushed, isTrue);
+    expect(isPopped, isTrue);
+  });
+
   group('error control test', () {
     testWidgets('onUnknownRoute null and onGenerateRoute returns null', (WidgetTester tester) async {
       final GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();