it's a good thing (that)
(also it's a good job (that) ) used when saying that there would have been problems if something had not happened

[TahlilGaran] Dictionary of Contemporary English ▲

lucky happening because of good luck, or bringing you good luck:
a lucky guess
Seven is considered a lucky number.
It’s lucky that I’ve got some spare keys.
Italy got a lucky goal in the last five minutes of the game.
‘How did you know he’d be there?’ ‘It was a lucky guess.’
fortunate happening because of good luck. Fortunate is more formal than lucky:
It was extremely fortunate that there was no one in the building when the bomb went off.
I’m in the fortunate position of doing a job I love.
Some plants actually prefer a lot of shade, which is fortunate for gardeners choosing plants for gloomy corners.
it’s a good thing (that) (also it’s a good job (that) British English) spoken used when saying that there would have been problems if something had not happened:
It’s a good thing that you brought an umbrella with you.
It’s a good job I’m here to help.
miraculous extremely lucky in a way that is almost unbelievable:
A teenager had a miraculous escape last night when the car she was travelling in overturned.
The doctor gave her a month to live but she made a miraculous recovery.
It was miraculous that no one was seriously injured in the accident.
fortuitous /fɔːˈtjuːətəs $ fɔːrˈtuː-, fɔːˈtjuːɪtəs $ fɔːrˈtuː-/ formal happening because of good luck:
a fortuitous decision
a fortuitous coincidence
It was fortuitous that no one else was hurt.
a fluke /fluːk/ informal something that happens by chance, not because of skill or good judgement:
The goal was a fluke.
By a fluke, he managed to get the question right.
be in the right place at the right time used when saying that someone is lucky and the situation is right for them:
Making money from buying property is easy – you just have to be in the right place at the right time.

[TahlilGaran] English Thesaurus ▲