[go_router] fix onExit ignored for GoRoute nested inside ShellRoute

[qiqetes]

onExit was silently ignored when the route was nested inside a ShellRoute + added test for async and sync pop for this specific case
Attempt to fix flutter/flutter#137829 flutter/flutter#137829

List which issues are fixed by this PR. You must list at least one issue.

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[gemini-code-assist]

Code Review

This pull request updates GoRouterDelegate to resolve the leaf match when handling a ShellRouteMatch by traversing nested matches to find the final leaf route. This ensures that the correct route and state are used when invoking the onExit callback. Additionally, synchronous and asynchronous regression tests have been added to verify back button behavior with ShellRoute. There are no review comments, and I have no feedback to provide.

[stuartmorgan-g]

Thanks for the contribution!

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[gemini-code-assist]

Code Review

This pull request updates GoRouterDelegate to correctly handle onExit callbacks for GoRoutes nested inside a ShellRoute by traversing to the leaf match, and adds corresponding regression tests. A review comment identifies a potential null pointer exception when force-unwrapping navigatorKey.currentContext inside scheduleMicrotask, as the context could be null if the widget tree is unmounted before the microtask executes, and suggests adding a defensive null check.

[gemini-code-assist]

Since onExit is executed asynchronously inside a scheduleMicrotask, there is a possibility that the widget tree has been unmounted or the navigator disposed by the time the microtask runs, making navigatorKey.currentContext null. Calling navigatorKey.currentContext! will then trigger a null check operator error.

Consider defensively checking if the context is null before proceeding, and if so, fallback to completing the route match:

    scheduleMicrotask(() async {
      final BuildContext? context = navigatorKey.currentContext;
      if (context == null) {
        _completeRouteMatch(result, match);
        return;
      }
      final bool onExitResult = await routeBase.onExit!(
        context,
        leafMatch.buildState(_configuration, currentConfiguration),
      );
      if (onExitResult) {
        _completeRouteMatch(result, match);
      }
    });