Objective-C Sheet Windows

Hello everyone...

I have been trying many times to get an Objective-C sheet window to work, but have not had any luck. This is the code that won't work for me:

Code:

    [NSApp beginSheet:configureSheet modalForWindow:mainWindow
        modalDelegate:self didEndSelector:NULL contextInfo:nil];

I've connected configureSheet and mainWindow to their respective windows in Interface Builder. Any ideas?

Thanks in advance.