使用嵌套块时出现BAD ACCESS

Question

问题在于：我需要从非UI线程创建警报。但是当我使用块时，我得到一个“BAD ACCESS”错误。 这是代码：

- (void)loginViewFetchedUserInfo:(FBLoginView *)loginView user:(id<FBGraphUser>)user
{       
        [currentUser updateWithFacebookUser:user errorBlock:^(void) {
            UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error"
                                                            message:@"Facebook login was unsuccessfult. It's already in the system." delegate:self cancelButtonTitle:@"Cancel"
                                                  otherButtonTitles:@"OK", nil];
            [alert show];
        }];
}

在updateWithFacebookUser中：errorBlock：

- (void)updateWithFacebookUser:(id<FBGraphUser>)facebookUser errorBlock:(void (^)(void))errorBlock
{
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        errorBlock();
    }];    
}

让我感到惊讶的是，当我在没有任何阻挡的情况下显示警报时，一切都还可以：

  - (void)loginViewFetchedUserInfo:(FBLoginView *)loginView user:(id<FBGraphUser>)user
    {       
            [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error"
                                                                message:@"Facebook login was unsuccessfult. It's already in the system." delegate:self cancelButtonTitle:@"Cancel"
                                                      otherButtonTitles:@"OK", nil];
                [alert show];
            }];
    }

此外，我注意到警报已显示，然后才会发生错误。可能是什么问题？