fix: avoid int pop payload in news filter bottom sheet

[Pankajbarad18]

Small fix: use Navigator.of(context).pop() instead of pop(0) to avoid returning an int when showCustomBottomSheet returns Future<String?>.

Fixes
Fixes #217

How to reproduce

    1. Launch app.
    2. Tap the news filter icon.
    3. Select a topic and tap Apply.
    4. Previously the app threw: TypeError: 0: type 'int' is not a subtype of type 'String?'.

What I changed

    - [ui.dart]: changed Navigator.of(context).pop(0) → Navigator.of(context).pop().

Testing

    - Manual: followed reproduction steps and verified no TypeError and navigation to the filtered NewsFeed works.

Notes for maintainers

    - Small, isolated change; no breaking behavior; ready to merge.