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But if it does happen, the self-driving airplane needs to have a landing plan that isn't "FALL* on a population center."*
Planes don't simply "fall" though, or more specifically it's not a limiting factor for autonomous flight because if they do Fall (due to total structural breakup) there is absolutely nothing a human pilot can do about it either. The more likely worst problem that is still within the realm of "anything can be done about it" is total engine failure (and with it the potential for failure of systems that depend upon engine power as well). Commercial jet aircraft tend to have glide ratios (how far they can go horizontally for each unit they drop vertically) of high single digits to low teens, though some go farther (the 747 for example is 17:1). So from 35000' say they'll typically be able to travel at least 60-110 miles (with some bonus for their initial velocity).
The power problem has also needed to be considered long since due to all the fly-by-wire systems, though for an autonomous cargo aircraft this may be another area worth a second look since some weight savings from normal life support might be well spent on additional redundancy.
As other posters have pointed out a fully autonomous aircraft just needs to aim for the most sparsely populated area of land or body of water. No need to worry about human life on the aircraft dramatically opens up potential ok ground areas. So a failure at cruising altitude that a piloted aircraft might survive anyway should leave a sizeable radius for an unpiloted aircraft to find a ditch point with low risk to human life. So ultimately while yeah sure, of course this should all be formalized, it shouldn't be a real blocker for this application.