Fix crash on rapid back press by checking back stack state before removal

[easternkite]

Summary

This PR fixes a crash that occurs when the back button is pressed rapidly while on certain screens (e.g. Create, About).

The issue was caused by backStack.removeLastOrNull() being called without validating the current top of the back stack, leading to IllegalArgumentException: NavDisplay backstack cannot be empty.

Changes

• Added type-checks before removing the top of the back stack to ensure the app is in a valid navigation state.
• Specifically checks if backStack.last() is of type Create or About before removing it.

[riggaroo]

thanks - this will be fixed in the next version of nav3 - so we wont need to implement this fix: https://issuetracker.google.com/419017866