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

[brotoo25]

GoRouterDelegate._findCurrentNavigators() force-unwraps each shell navigator's state (walker.navigatorKey.currentState!) while walking the current configuration. When the system back button is pressed while currentConfiguration references a ShellRoute whose navigator is not mounted — a transient state during redirects away from a shell or when resuming from background — popRoute() throws Null check operator used on a null value and crashes the app (observed in production via crash reporting on go_router 17.3.0, Android).

This applies the same null-tolerant handling that canPop() in the same file already uses for this state: if a shell navigator along the chain is not mounted, stop walking — nothing below it can be mounted either. This mirrors the existing early-break for pageless routes in the same loop, and the root navigator in this method is already null-guarded in the same way.

The added regression test pins the transient state deterministically (configuration pointing at a shell route while the widget tree shows a non-shell route) and fails on main with the exact production stack trace before this change.

Related to, but distinct from, flutter/flutter#187616 and #11911: those cover currentConfiguration.matches being empty (Bad state: No element); this PR fixes the non-empty case where a shell navigator is unmounted. The two changes are independent and compatible.

Fixes flutter/flutter#188993

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request addresses a potential crash in popRoute() within GoRouterDelegate when a shell route's navigator is not mounted. The implementation replaces a force-unwrap operator with a null check, breaking the traversal loop if the navigator state is null. A widget test simulating this scenario has been added to prevent regressions, and a pending changelog entry has been created. There are no review comments, and I have no feedback to provide.

[chunhtai]

looking at the issue, do you have a reproducible code that show case the transient state?

also I am not too sure what may be the correct behavior when the backbutton is pressed during that state. It felt wrong to pop the parent navigator because child navigator is in transient state. Perhaps the correct solution will be disallow the current configuration to be in such state