if you are trying to access the rootViewController
you set in your appDelegate. try this:
Objective-C
YourViewController *rootController = (YourViewController*)[[(YourAppDelegate*)
[[UIApplication sharedApplication]delegate] window] rootViewController];
Swift
let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate
let viewController = appDelegate.window!.rootViewController as YourViewController
Swift 3
let appDelegate = UIApplication.shared.delegate as! AppDelegate
let viewController = appDelegate.window!.rootViewController as! YourViewController
Swift 4 & 4.2
let viewController = UIApplication.shared.keyWindow!.rootViewController as! YourViewController
Swift 5 & 5.1 & 5.2
let viewController = UIApplication.shared.windows.first!.rootViewController as! YourViewController