fix: 🐛 Handle null RenderBox in AnchoredOverlay and provide fallback values(#464)

[Sahil-Simform]

fix: 🐛 Handle null RenderBox in AnchoredOverlay and provide fallback values(#464)

Description

This PR addresses a bug where AnchoredOverlay was causing crashes when RenderBox was null or not yet laid out.

Added null-safe checks for RenderBox in AnchoredOverlay.
Provided fallback values (Offset.zero and an empty Rect) for safe calculations.
Improved code readability with null-aware operators and comments.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #464