Centering a document window

Question

I am writing a core data, document based app. I am trying to center the document's window. I have tried calling the following code from the document's makeWindowControllers method, from the window controller's awakeFromNib method and from the window controller's showWindows method. I keep getting a nil result for myWindow. Any ideas? Thank you.

NSWindow *myWindow = [myWindowController window];
[myWindow center];