Guard against a null navigator on popping a route. (#49329)

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();
     });
   }
 

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>();