Well, in case someone else will face the same issue, I've just updated ko.mapping plugin javascript as follows:
...
if (hasUpdateCallback() && mappedRootObject) {
mappedRootObject(updateCallback(mappedRootObject));
}
...
So, I check for existense of mappedRootObject before updating it. Although it worked fine, I've figured out a way, how to avoid update callback in my project at all, so this fix became useless to me, unfortunately.