allow passing textcontroller to FlutterMentions

[Fnalsk]

I want something like this:

FlutterMentions(
  key: _mentionsKey,
  decoration: InputDecoration(
    suffixIcon: ValueListenableBuilder(
      valueListenable: _mentionsKey.currentState!.controller!,
      builder: (BuildContext context, value, Widget? child) => // some widget
    )
  ),
  mentions: [
    ...
  ],
)

Essentially I need the controller to be used in the decoration. The problem with this is that the InputDecoration builds first, so _mentionsKey.currentState! throws an exception saying null check operator used on null value. This wouldn't be a problem if I could just build the TextEditingController (or AnnotationEditingController) elsewhere, then simply pass it to this widget.