saved by the bell idiom
救命钟（某种困难局面突然结束时所说的措辞，否则你就得做或说你不想做或说的事）
something that you say when a difficult situation ends suddenly before you have to do or say something that you do not want to
Luckily my bus arrived before I had time to reply. Saved by the bell. 正当我要回答时，我等的公交车就到了，真是万幸。