Fix crash when inserting newlines rapidly

[bilaldev05]

This PR fixes an assertion failure in editable_box.dart when rapidly inserting new lines.

Under fast input, the document model can grow faster than the render tree updates, causing childAtPosition() to return null.

Instead of asserting, we now gracefully fallback to the last available child.

Steps to reproduce:

1. Open demo app
2. Hold Enter to insert many new lines
3. Observe assertion error in console

After fix:
No assertion occurs and scrolling works normally.

Tested on:
Android (Pixel Fold)
Flutter 3.41.0