We were failing to dispose of animation controllers created by `animateTo` when `didUpdateWidget` happens and we null out the `_attachedController`. This would cause the listener added here to fail on a null assertion:

https://github.com/flutter/flutter/blob/fef41cfce0e3582907f6846cf2385470bb17ecd1/packages/flutter/lib/src/widgets/draggable_scrollable_sheet.dart#L135

Without the code change, the test ends up throwing exceptions related to that line. I don't have a great way to verify what this does visually though, hoping for help on that from internal customer.

b/279930163

See cl/527868355 as well.

Fixes https://github.com/flutter/flutter/issues/125709