We have a guess.
Well it is a bit hard when we only have a stack-trace from your code but one guess could be that we have changed the REST API for devices not to included the child items per default.
If that is the case you should include this in your GET request:
?includeChildren=settings
I don’t see that it has changed for the documentation notes, but if my guess is correct you might find some extra information on this topic: RestAPI incomplete