NEW YORK/PARIS (Reuters) – An airline’s warning of a “bubble” in the market for used jetliners shows how vulnerable Boeing Co (BA.N) may be because of slowing sales of its widebody 777 jets, and fueled speculation that it may have to cut the plane’s production rate. Boeing, which is due to report its third-quarter earnings next Wednesday, needs to secure orders for about 200 more 777s, at its current rate of production, before its successor, the 777X, enters service in 2020.