setBusy method
Called by blocs (subclasses) when the updated state isn't available immediately
This should be used when a bloc is waiting for an async operation (e.g. a network call)
error is cleared whenever isBusy is set
Optional argument event
is the name of event which is calling setError
Implementation
@protected
void setBusy({String? event}) {
if (_isBusy) return;
_monitor.onBusy(runtimeType.toString(), event: event);
_isBusy = true;
_error = null;
_stream?.add(null);
}