-[UIApplication delegate] must be called from main thread only

In Swift, you could also use DispatchQueue.main.async to call the UI controlling method from the main thread

DispatchQueue.main.async {
    YourUIControlMethod()
}

Just call it from the main thread like this.

Objective-C

dispatch_async(dispatch_get_main_queue(), ^{
  [[UIApplication delegate] fooBar];
});

Swift

DispatchQueue.main.async {
  YourUIControlMethod()
}

Reaching out to your app delegate like this, is a hint that your architecture could use a little cleanup.

You can call delegates from any thread you want. You only need to make sure you're on the main thread for UIKit calls. Or that you're on the correct thread your CoreData objects expect. It all depends on the API contract your objects have.