FirebaseFacebookAuthUI/Sources/FUIFacebookAuth.m [252:273]:
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                                  completion:^(FIRAuthCredential *_Nullable credential,
                                               NSError *_Nullable error) {
    if (error) {
      [FUIAuthBaseViewController showAlertWithMessage:error.localizedDescription
                             presentingViewController:presentingViewController];
      if (completion) {
        completion(nil, error, nil, nil);
      }
      return;
    }
    if (completion) {
      UIActivityIndicatorView *activityView =
          [FUIAuthBaseViewController addActivityIndicator:presentingViewController.view];
      [activityView startAnimating];
      FIRAuthResultCallback result = ^(FIRUser *_Nullable user,
                                       NSError *_Nullable error) {
        [activityView stopAnimating];
        [activityView removeFromSuperview];
      };
      completion(credential, nil, result, nil);
    }
  }];
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FirebaseOAuthUI/Sources/FUIOAuth.m [324:345]:
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                                    completion:^(FIRAuthCredential *_Nullable credential,
                                                 NSError *_Nullable error) {
      if (error) {
        [FUIAuthBaseViewController showAlertWithMessage:error.localizedDescription
                               presentingViewController:presentingViewController];
        if (completion) {
          completion(nil, error, nil, nil);
        }
        return;
      }
      if (completion) {
        UIActivityIndicatorView *activityView =
            [FUIAuthBaseViewController addActivityIndicator:presentingViewController.view];
        [activityView startAnimating];
        FIRAuthResultCallback result = ^(FIRUser *_Nullable user,
                                         NSError *_Nullable error) {
          [activityView stopAnimating];
          [activityView removeFromSuperview];
        };
        completion(credential, nil, result, nil);
      }
    }];
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