There are valid cases where a module returns None. E.g. no images were provided for LSC calibration. We should however define proper semantics there. Continue with a warning for now. Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>