[go_router] Fix: Consistent PopScope Handling on Root Routes issue #140869

[omar-hanafy]

Description:
This PR addresses issue #140869 related to back button handling on root routes in GoRouterDelegate. The current _findCurrentNavigator() function only assigns NavigatorState when it can pop, which prevents PopScope and custom back button logic from triggering on root routes, especially on Android. Additionally, the onExit callbacks on routes requiring custom exit handling are affected.

What This PR Changes:

1. Modification of _findCurrentNavigator():

   • Removed the conditional canPop check, setting state = navigatorKey.currentState; directly to ensure NavigatorState is always non-null, allowing consistent PopScope handling on root routes.

2. Adjustment of popRoute() to Preserve onExit Logic:

   • Updated popRoute() to call maybePop() unconditionally to make sure PopScope can handle back button actions on root pages.
   • Added fallback to onExit logic in popRoute() if maybePop() does not handle the pop, this way, any route-specific exit callbacks are still functional.

Impact:

• Resolves Inconsistent Back Button Handling: The fix enables PopScope and back button handling on root pages without affecting nested routes or shell routes.
• Maintains onExit Callback Functionality: onExit is invoked when maybePop() doesn’t handle the pop, preserving custom exit behaviors.

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Pre-launch Checklist

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[omar-hanafy]

Hi @chunhtai,

This PR is ready for your review. All checks have passed, and there are no issues reported. When you have a moment, could you please take a look and provide an approval if everything looks good? Your review is the last step for merging.

Thank you!