Interesting. My main project uses YEP's CTB and I haven't encountered this, though I am probably not using the latest version.
Just looking at what the error is, it's looking at a "null" (in other words, no data) and trying to treat it as a skill/item. So somewhere along the way, it's getting an invalid skill. But I don't have enough info to pin down where from this alone, I'm afraid.
Perhaps show me exactly what you're doing when this bug occurs? It would make it easier to track down.