What is the fancy way to use SnackBar in StreamBuilder?

First, you must ensure that you are always returning a widget

and then you can schedule the SnackBar for the end of the frame

if(state is AuthUnauthenticated){
  WidgetsBinding.instance.addPostFrameCallback((_) => _showErrorMessage(state.errorMessage));
  return Container();
}

You should also check if the data is null o the snapshot has data.