Use StreamController.isClosed to check if the controller is closed or not, if not closed add data to it.
if (!_controller.isClosed)
_controller.sink.add(...); // safe to add data as _controller isn't closed yet
From Docs:
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.