First you made appDelegate
point to [[UIApplication sharedApplication] delegate]
. Then you made it point to nowhere. But [[UIApplication sharedApplication] delegate]
remains the same. You did nothing with it. You didn't even touch it.
What you probably are thinking of is:
[[UIApplication sharedApplication] setDelegate:nil];
But it also shouldn't produce warnings or crashes. It should only result in not calling app delegate methods since messages sent to nil do nothing.