windows are expected to have a root view controller at the end of application launch

windows are expected to have a root view controller at the end of application launch

This is a warning that the compiler shows, so the application works. It is caused  because you have added a subview to a view without declaring the root View Controller first. To fix it you must replace:

[windows addSubview:self.aViewController.view];

for:

[windows setRootViewController:self.aViewController];