incorporate safe fallback destination to fix null destination causing UI flickering

[trambui09]

Fixes #1675

When navigating to a destination lower in the hierarchy, the currentBackStackEntry temporarily becomes null due to the incremental update of the back stack by the NavController. This transient state causes UI elements, such as the Top Bar, to be removed and re-added, leading to a noticeable flicker.

A previousDestination fallback mechanism has been introduced. This approach ensures the UI maintains its state by retaining the last known valid destination when currentBackStackEntry is null. The following changes were made:

• Added a previousDestination property that references the most recent non-null destination.
• Updated the currentTopLevelDestination logic to use previousDestination to determine the active destination, ensuring the UI remains stable during transitions.

NiA-currentDestination-null-bug.mp4

[dturner]

Thank you for investigating and fixing this 👍