[go_router] Fix popRoute crash when a shell route's navigator is not mounted

packages/go_router/lib/src/delegate.dart

@@ -118,7 +118,13 @@ class GoRouterDelegate extends RouterDelegate<RouteMatchList> with ChangeNotifie
 
     RouteMatchBase walker = currentConfiguration.matches.last;
     while (walker is ShellRouteMatch) {
-      final NavigatorState potentialCandidate = walker.navigatorKey.currentState!;
+      final NavigatorState? potentialCandidate = walker.navigatorKey.currentState;
+      if (potentialCandidate == null) {
+        // Stop if the shell route's navigator is not mounted, e.g. when the
+        // configuration references a shell route whose widget tree has not
+        // been built yet or has already been removed.
+        break;
+      }
 
       final ModalRoute<dynamic>? modalRoute = ModalRoute.of(potentialCandidate.context);
       if (modalRoute == null || !modalRoute.isCurrent) {

packages/go_router/pending_changelogs/fix_poproute_unmounted_shell_navigator.yaml

@@ -0,0 +1,5 @@
+changelog: |
+  - Fixes `popRoute()` crashing with "Null check operator used on a null value"
+    when the current configuration references a shell route whose navigator is
+    not mounted.
+version: patch

packages/go_router/test/delegate_test.dart

@@ -343,6 +343,30 @@ void main() {
       ], tester);
       expect(await goRouter.routerDelegate.popRoute(), isFalse);
     });
+
+    testWidgets('popRoute does not throw when a shell navigator in the configuration '
+        'is not mounted', (WidgetTester tester) async {
+      final GoRouter goRouter = await createRouter(<RouteBase>[
+        GoRoute(path: '/', builder: (_, _) => const Text('Home')),
+        ShellRoute(
+          builder: (_, _, Widget child) => child,
+          routes: <RouteBase>[
+            GoRoute(path: '/dashboard', builder: (_, _) => const Text('Dashboard')),
+          ],
+        ),
+      ], tester);
+
+      // Simulate the transient state where the configuration references a
+      // shell route whose navigator is not (or no longer) mounted, e.g. while
+      // the widget tree rebuilds during a redirect or after resuming from
+      // background.
+      goRouter.routerDelegate.currentConfiguration = goRouter.configuration.findMatch(
+        Uri.parse('/dashboard'),
+      );
+
+      // Should not throw "Null check operator used on a null value".
+      expect(await goRouter.routerDelegate.popRoute(), isFalse);
+    });
   });
 
   group('push', () {