Where are you calling this method from? I had an issue where I was attempting to present a modal view controller within the viewDidLoad method. The solution for me was to move this call to the viewDidAppear: method.
My presumption is that the view controller’s view is not in the window’s view hierarchy at the point that it has been loaded (when the viewDidLoad message is sent), but it is in the window hierarchy after it has been presented (when the viewDidAppear: message is sent).
Caution
If you do make a call to presentViewController:animated:completion: in the viewDidAppear: you may run into an issue whereby the modal view controller is always being presented whenever the view controller’s view appears (which makes sense!) and so the modal view controller being presented will never go away…
Maybe this isn’t the best place to present the modal view controller, or perhaps some additional state needs to be kept which allows the presenting view controller to decide whether or not it should present the modal view controller immediately.