in lib/src/multi_channel.dart [233:244]
void _closeInnerChannel() {
_inner!.sink.close();
_innerStreamSubscription!.cancel();
_inner = null;
// Convert this to a list because the close is dispatched synchronously, and
// that could conceivably remove a controller from [_controllers].
for (var controller in List.from(_controllers.values)) {
controller.local.sink.close();
}
_controllers.clear();
}