If `error` remains null, calling `localizedDescription()` will crash. Add a null check for `error` before dereferencing.

[freelw]

If error remains null, calling localizedDescription() will crash. Add a null check for error before dereferencing.

        if (error) {
            std::cerr << "Error creating compute pipeline state: " << error->localizedDescription()->utf8String() << std::endl;
        } else {
            std::cerr << "Error creating compute pipeline state: Unknown error." << std::endl;
        }

Originally posted by @Copilot in #28 (comment)