iOS: clientId.refreshToken cannot be null.

I want to keep logging in, but I understand that the accessToken is only valid for up to an hour, and I also understand that I need a refreshToken. But I don't know how to get a refreshToken, it seems that GoogleSignInAuthentication only has accessToken and idToken. Can someone please advise me on this?

Also, the word seems to be wrong
```
final refreshToken = credentials.refreshToken;
if (refreshToken == null) {
throw ArgumentError('clientId.refreshToken cannot be null.');
}
```
-> credentials.refreshToken cannot be null.