onSend property
Called when the user taps send. replyTo mirrors replyTarget at the
moment of sending: null means "post a new top-level comment",
non-null means "reply to this item". The sheet clears its own text
field after calling this; you're responsible for clearing any of your
own state (e.g. attached media) and persisting the comment.
Implementation
final void Function(String text, T? replyTo)? onSend;