Updated Bottom NavigationBar with NullSafety

[rahul31124]

This pull request addresses a critical issue where the app was crashing when a user attempted to access the profile section from the bottom navigation while not logged in.

Issue Fixed:
The app was previously crashing when a user clicked on the profile option in the bottom navigation, and the user was not logged in.

Changes Implemented:
Null Safety: Added null and blank checks for the username to ensure that the app only tries to navigate to the profile if the username is valid (not null or blank).

User Experience Improvement: If the user is not logged in and attempts to navigate to the profile, an AlertDialog will appear prompting the user to log in, instead of the app crashing. The dialog provides the option to either log in or cancel the action.

Before

Before.mp4

After

After.mp4

[rahul31124]

@07jasjeet need your help

[07jasjeet]

Hey @rahul31124, you can reach out to the team by going through the communication section in readme. Just tag me there, my nick is "jasje".