If you need to handle it inside the function differently, then it is straightforward:

(args.getOrNull(0) ?: error("Missing arg"))
    .toIntOrNull() ?: error("Expected int"))

However, if you need to represent this variety of errors in the result of the function, then null is clearly not enough. You’ll need a different approach. Fortunately, this need to represent more details in the result happens exceedingly rare in practice. A single bit of information (result/no result) is enough for most simple functions (and we do strive to keep our functions simple) and the null reference as “no result” indicator is perfect fit to represent it.