isClosed property Null safety
Whether the stream controller is closed for adding more events.
The controller becomes closed by calling the close method. New events cannot be added, by calling add or addError, to a closed controller.
If the controller is closed, the "done" event might not have been delivered yet, but it has been scheduled, and it is too late to add more events.
@override bool get isClosed => _controller.isClosed;