我发现了解决方案，并真正希望这对某人有帮助...

I discovered the solution and truly hope this will help someone...

        if (success) 
        {   

            if ([parser.currentValue isEqualToString:@"G"])

            // Make the changes here - start
            dispatch_queue_t main_queue = dispatch_get_main_queue();
            dispatch_async(main_queue, ^{
            thirdView.image = greenTick.jpg;
            [secondView addSubview:thirdView];
            });
            // Make the changes here - end
        } 
        else 
        {
            NSLog(@"Error parsing document!");
        }
} 
// Remove performSelectorOnMainThread