What is an idiom for "a supervening act that renders a course of action unnecessary"?

Solution 1:

I’m confused as to whether the phrase you are seeking should describe the lethargic inactivity, or the reprieve provided by the supervening event? – pbasdf

@pbasdf— The latter – user405662

How about saved by the bell?

Solution 2:

A fancy term for this is deus ex machina

... a seemingly unsolvable problem in a story is suddenly and abruptly resolved by an unexpected and unlikely occurrence Wikipedia

Also

"you got lucky"

"that was a (lucky) fluke"

"you had a lucky break"

"that was an unwarranted success"

"luck was on his side"

"he lives a charmed life"

Solution 3:

Person 1 to Person 2: "Well, it's OBE now."

OBE = Overcome by events.