Adding webtrends framework crash the app

Try to add webtrends frameworks to one of my app. The framework request user to modify the app so that applicationDelegate should subclass webtrends' delegate. After the code is changed, the app crash right before the app launch:

    fwXXXXX[10818:60b] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<fwXXXXXAppDelegate 0xc574d90> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key window.' 

The app crashed because it cannot set the key window when loading the nib.

It turns out it didn't work because I'm using auto-synthesise on the property window.

So I added:

   @synthesize window = _window;

to the implementation and now it works fine.