Present view controller from app delegate

You can also try:

[[[UIApplication sharedApplication] keyWindow] rootViewController]

How I use it:

#define ROOTVIEW [[[UIApplication sharedApplication] keyWindow] rootViewController]
[ROOTVIEW presentViewController:interstitialViewController animated:YES completion:^{}];

Swift 3 version for checked response :

UIApplication.shared.keyWindow?.rootViewController

Swift 5 and up

Check this:

UIApplication.shared.windows.first?.rootViewController