Previously we didn't distinguish between canceling the menu (which
popped the route with no return value, i.e. null) and explicitly
selecting an item whose value is null (pop with value null). Both fired
onChange(null).

Now we box the return value so we can distinguish the two cases.

I would have preferred to just disallow "null" as a value but it turns
out people like being able to use "null" as a value for much the same
reason we do, and it seems best for us to pay the complexity cost of
boxing than having everyone else do it. :-)