fix: Scrollable of context for nullable instance

[adrianostas]

Replace the Scrollable.of(context) with Scrollable.maybeOf(context) (after updating to Flutter3.7.0 tests are failing).

[ueman]

This fixes #402. You also need to raise the minimal Flutter version to 3.7 since the Scrollable.maybeOf(context) was introduced in 3.7 and thus does not work on older Flutter versions.

[letsar]

Thank you for this PR, I'll close it since this has already been fixed.