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

This warning leads to a serious problem cause I really can't call the delegate outside of the main thread using Xcode 9 beta 2. Strange thing is that this was working when I was using Xcode 8.3.3.

Also I thought it would only be good practice to call delegates from main thread only, isn't it? So why is this causing the app to crash now?

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.

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

DispatchQueue.main.async {
    YourUIControlMethod()
}