class xxx is being deallocated while key value observers are still registered with it

FROM : Steve Nicholson
DATE : Thu Mar 27 18:46:18 2008

In preparation for supporting multiple windows for a single document, 
I modified a document-based app I'm working on to have it use a 
subclass of NSWindowController. Everything is working fine but when I 
close a window, the message "An instance 0x306860 of class Problem is 
being deallocated while key value observers are still registered with 
it." is appearing in the log. The instance of the class Problem is in 
my NSDocument-based class and the NSWindowController sub-class 
accesses that.

The bindings are all set up in Interface builder. The class of the 
File's Owner in the controller nib is correctly set to (the very 
cleverly-named) MainWindowController.

I suppose I can implement windowWillClose in MainWindowController to 
unbind everything before the window closes, but since the bindings 
are set up in IB, the asymmetry makes me think I'm doing something 
wrong.

If anyone can point out something I'm doing wrong, I'd appreciate it.