fix: 🐛 null-check crash in ShowcaseController._initRootWidget when the Showcase is unmounted in the same frame it mounts (#659)

[apurva010]

Description

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 #659

[dmallo97]

Thanks for the fix! There's a second unguarded _context access on the same path: scrollIntoView awaits Scrollable.ensureVisible and then calls _waitForScrollToSettle, whose first line is Scrollable.maybeOf(_context). If the target is unmounted during the scroll (e.g. with enableAutoScroll: true), it throws the same null-check error in release. The same guard at the top of _waitForScrollToSettle would cover it:

Future<void> _waitForScrollToSettle({...}) async {
  if (!_mounted) return;
  final scrollableState = Scrollable.maybeOf(_context);