[go_router] pop() not working when both top-level 'onEnter' and 'redirect' are async and the route have onExit callback

### What package does this bug report belong to?

go_router

### What target platforms are you seeing this bug on?

Android, iOS

### Have you already upgraded your packages?

Yes

### Dependency versions

<details><summary>pubspec.lock</summary>

```lock
# Generated by pub
# See https://dart.dev/tools/pub/glossary#lockfile
packages:
  characters:
    dependency: transitive
    description:
      name: characters
      sha256: f71061c654a3380576a52b451dd5532377954cf9dbd272a78fc8479606670803
      url: "https://pub.dev"
    source: hosted
    version: "1.4.0"
  collection:
    dependency: transitive
    description:
      name: collection
      sha256: "2f5709ae4d3d59dd8f7cd309b4e023046b57d8a6c82130785d2b0e5868084e76"
      url: "https://pub.dev"
    source: hosted
    version: "1.19.1"
  flutter:
    dependency: "direct main"
    description: flutter
    source: sdk
    version: "0.0.0"
  flutter_web_plugins:
    dependency: transitive
    description: flutter
    source: sdk
    version: "0.0.0"
  go_router:
    dependency: "direct main"
    description:
      name: go_router
      sha256: eff94d2a6fc79fa8b811dde79c7549808c2346037ee107a1121b4a644c745f2a
      url: "https://pub.dev"
    source: hosted
    version: "17.0.1"
  logging:
    dependency: transitive
    description:
      name: logging
      sha256: c8245ada5f1717ed44271ed1c26b8ce85ca3228fd2ffdb75468ab01979309d61
      url: "https://pub.dev"
    source: hosted
    version: "1.3.0"
  material_color_utilities:
    dependency: transitive
    description:
      name: material_color_utilities
      sha256: f7142bb1154231d7ea5f96bc7bde4bda2a0945d2806bb11670e30b850d56bdec
      url: "https://pub.dev"
    source: hosted
    version: "0.11.1"
  meta:
    dependency: transitive
    description:
      name: meta
      sha256: "23f08335362185a5ea2ad3a4e597f1375e78bce8a040df5c600c8d3552ef2394"
      url: "https://pub.dev"
    source: hosted
    version: "1.17.0"
  sky_engine:
    dependency: transitive
    description: flutter
    source: sdk
    version: "0.0.0"
  vector_math:
    dependency: transitive
    description:
      name: vector_math
      sha256: d530bd74fea330e6e364cda7a85019c434070188383e1cd8d9777ee586914c5b
      url: "https://pub.dev"
    source: hosted
    version: "2.2.0"
sdks:
  dart: ">=3.9.0 <4.0.0"
  flutter: ">=3.35.0"
```

</details>


### Steps to reproduce

**Using the provided code:**
1. tap "Go to Details" button ( context.push() )
2. tap "Back" button  ( context.pop() )
3. page is not popped


### Things to note:
- both declared routes are at the same level
- navigation is done via `context.push()` and `context.pop()`, not `context.go()`

### Problem:

This problem appears only when both top-level `onEnter` and top-level `redirect` callbacks return a `Future` AND the route you try to pop has an `onExit` callback (async or sync).

In my example, the bug disappears if you either:
- remove the `onEnter`
- make `onEnter` synchronous
- remove the `redirect`
- make `redirect` synchronous
- remove the `onExit` callback from the route being pop

Also keep in mind that `go_router_builder` systematically creates an `onExit` callback (that returns `true`) for the generated routes, even when no override is provided, which is different from the behavior of a "manual" `GoRoute`, because not giving an `onExit` callback to `GoRoute` the constructor will lead to `onExit` being `null`.

So this problem can pop when migrating to `go_router_builder`, because it will add a default `onExit` callbacks to GoRoute that previously had a null `onExit`.

### Expected results

When tapping the "Back" button on the Details page, it should pop the current route and show the Home page

### Actual results

When tapping the "Back" button on the Details page, the route is not popped and nothing happens.
If I tap again, an Exception is thrown. (see the stacktrace below)

### Code sample

<details open><summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';
import 'package:go_router/go_router.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(_) {
    return MaterialApp.router(
      routerConfig: GoRouter(
        debugLogDiagnostics: true,
        onEnter: (_, __, ___, ____) => Future.value(const Allow()), // <=======
        redirect: (_, __) => Future.value(null), // <=======
        routes: <RouteBase>[
          GoRoute(
            path: '/',
            builder: (_, __) => const HomePage(),
          ),
          GoRoute(
            path: '/details',
            builder: (_, __) => const DetailsPage(),
            onExit: (_, __) => Future.value(true), // <=======
          ),
        ],
      ),
    );
  }
}

class HomePage extends StatelessWidget {
  const HomePage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Text('Home page'),
            ElevatedButton(
              onPressed: () => context.push('/details'), // <=======
              child: const Text('Go to Details'),
            ),
          ],
        ),
      ),
    );
  }
}

class DetailsPage extends StatelessWidget {
  const DetailsPage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Text('Details page'),
            ElevatedButton(
              onPressed: () => context.pop(), // <=======
              child: const Text('Back'),
            ),
          ],
        ),
      ),
    );
  }
}
```

</details>


### Screenshots or Videos

<details open>
<summary>Screenshots / Video demonstration</summary>

Not working: 
https://github.com/user-attachments/assets/5f0c78e9-4547-4526-b7b2-58ada099c6e8

Working (no onEnter OR no redirect): 
https://github.com/user-attachments/assets/9a03915b-30a0-4ae3-aac6-f614765eaaba
</details>


### Logs

<details open><summary>Logs</summary>

Launching the app and tapping "Go To Details" button:
```console
[GoRouter] Full paths for routes:
           ├─/ (HomePage)
           └─/details (DetailsPage)
[GoRouter] setting initial location null
[GoRouter] Using MaterialApp configuration
[GoRouter] pushing /details
```


Pressing "Back" button on Details page (page is not popped):
```console
[GoRouter] popping /
[GoRouter] restoring /
```


Pressing a second time the "Back" button:
```console
[GoRouter] popping /
[GoRouter] restoring /
[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: Bad state: Future already completed
#0      _AsyncCompleter.complete (dart:async/future_impl.dart:97:31)
#1      ImperativeRouteMatch.complete (package:go_router/src/match.dart:489:15)
#2      GoRouterDelegate._completeRouteMatch (package:go_router/src/delegate.dart:187:14)
#3      GoRouterDelegate._handlePopPageWithRouteMatch.<anonymous closure> (package:go_router/src/delegate.dart:167:9)
<asynchronous suspension>
```

</details>


### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
[✓] Flutter (Channel stable, 3.38.4, on macOS 26.2 25C56 darwin-arm64, locale en-FR) [7.4s]
    • Flutter version 3.38.4 on channel stable at /Users/REDACTED/fvm/versions/3.38.4
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 66dd93f9a2 (2 weeks ago), 2025-12-03 14:56:10 -0800
    • Engine revision a5cb96369e
    • Dart version 3.10.3
    • DevTools version 2.51.1
    • Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations, enable-native-assets, omit-legacy-version-file, enable-lldb-debugging

[✓] Android toolchain - develop for Android devices (Android SDK version 36.1.0) [9.7s]
    • Android SDK at /Users/REDACTED/Library/Android/sdk
    • Emulator version 36.2.12.0 (build_id 14214601) (CL:N/A)
    • Platform android-36, build-tools 36.1.0
    • Java binary at: /Library/Java/JavaVirtualMachines/openjdk-17.jdk/Contents/Home/bin/java
      This JDK is specified in your Flutter configuration.
      To change the current JDK, run: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment Homebrew (build 17.0.13+0)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 26.2) [13.3s]
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 17C52
    • CocoaPods version 1.16.2

[✓] Chrome - develop for the web [11ms]
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Connected device (4 available) [8.6s]
    • iPhone 15 Pro (iOS 26.2) (mobile) • 8630355B-90A8-450F-A036-C213C21DC870 • ios            • com.apple.CoreSimulator.SimRuntime.iOS-26-2 (simulator)
    • iPhone 15 Pro (mobile)            • 3D789230-DF92-4EF7-89E6-1E9072D77DEA • ios            • com.apple.CoreSimulator.SimRuntime.iOS-26-1 (simulator)
    • macOS (desktop)                   • macos                                • darwin-arm64   • macOS 26.2 25C56 darwin-arm64
    • Chrome (web)                      • chrome                               • web-javascript • Google Chrome 140.0.7339.133

[✓] Network resources [281ms]
    • All expected network resources are available.

• No issues found!
```

</details>


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Uh oh!

[go_router] pop() not working when both top-level 'onEnter' and 'redirect' are async and the route have onExit callback #180002

What package does this bug report belong to?

What target platforms are you seeing this bug on?

Have you already upgraded your packages?

Dependency versions

Steps to reproduce

Things to note:

Problem:

Expected results

Actual results

Code sample

Screenshots or Videos

Logs

Flutter Doctor output

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Uh oh!

[go_router] pop() not working when both top-level 'onEnter' and 'redirect' are async and the route have onExit callback #180002

Description

What package does this bug report belong to?

What target platforms are you seeing this bug on?

Have you already upgraded your packages?

Dependency versions

Steps to reproduce

Things to note:

Problem:

Expected results

Actual results

Code sample

Screenshots or Videos

Logs

Flutter Doctor output

Metadata

Metadata

Assignees

Labels

Type

Fields

Projects

Milestone

Relationships

Development

Issue actions