[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

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[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.