When passing the --validate option to fast downward and instead of PDDL files, giving an output.sas file as input, the driver is calling validate with the following options:
validate auto-detected-domain.pddl output.sas sas_plan
Obviously validate complains when trying to read the output.sas file.
It would be good if the driver instead complained that the --validate option and an output.sas file as input are incompatible.
The easiest way to achieve this would probably be to use the _looks_like_search_input() function from arguments.py to check that the input file is not PDDL before calling validate at the bottom of run_components.py.