I am getting following warning when I try to present a ActivityController on navigation controller,
Attempt to present 
         Replace line:
UIViewController *vc = self.view.window.rootViewController;
[vc presentViewController: activityController animated: YES completion:nil];
//to
[self presentViewController:activityController animated: YES completion:nil];
or
[self.navigationController pushViewController:activityController animated:YES];
Analysis: Because the present modal view ViewController class has not been loaded into the window. This is equivalent to the building, second floor haven't built, directly go to cover 3 floor, this is definitely not. Only after load ViewController's view ;
Python
- (void)viewDidAppear:(BOOL)animated {
 [super viewDidAppear:animated];
    [self showAlertViewController];
}
- (void)showAlertViewController {
    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"Hello world" message:@"(*  ̄3)(ε ̄ *)d" preferredStyle:UIAlertControllerStyleAlert];
    // Create the actions.
    UIAlertAction *cancelAction = [UIAlertAction actionWithTitle:@"hello" style:UIAlertActionStyleCancel handler:^(UIAlertAction *action) {
        NSLog(@"The \"Okay/Cancel\" alert's cancel action occured.");
    }];
    UIAlertAction *otherAction = [UIAlertAction actionWithTitle:@"world" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action) {
        NSLog(@"The \"Okay/Cancel\" alert's other action occured.");
    }];
    // Add the actions.
    [alertController addAction:cancelAction];
    [alertController addAction:otherAction];
    UIWindow *windows = [[UIApplication sharedApplication].delegate window];
    UIViewController *vc = windows.rootViewController;
    [vc presentViewController:alertController animated: YES completion:nil];
}
This's worked for me.
You are trying to present a view controller from the rootViewController. In your case I think the rootViewController is not the current ViewController. Either you presented or pushed a new UIViewController on top of it. You should present a view controller from the top most view controller itself.
You need to change:
UIViewController *vc = self.view.window.rootViewController;
[vc presentViewController: activityController animated: YES completion:nil];
to:
[self presentViewController: activityController animated: YES completion:nil];
I faced the similar issue in iPhone 6+.
In Swift 2.0
let obj = self.storyboard?.instantiateViewControllerWithIdentifier("navChatController") as! UINavigationController
obj.modalPresentationStyle = .FormSheet
obj.modalTransitionStyle = .CoverVertical
Constants.APPDELEGATE.window?.rootViewController?.presentViewController(obj, animated: false, completion: nil)
I solved it by this way.