Disable First Responder Menu Items?

[Darkroom]

when i switch from my Window Mode window to my Full Screen Mode window, i can disable "Minimize" and "Zoom"... but when i have a Sheet visible over the Window (in Window Mode) it doesn't allow me to disable "Minimize" and "Zoom" (they stay enabled)...

i don't understand why? i've set an outlet for the sheet in App Controller and connected it, so i'm thinking it's because these functions are first responder functions and that's why... do i have to set a delegate or something like that? (i still don't really understand what "Delegate" means )

im my appController.m

#pragma mark Disable Menu Items

// Tag #1:  Window Mode
// Tag #2:  Full Screen Mode
// Tag #4:  Show/Hide Toolbar
// Tag #5:  Minimize
// Tag #6:  Zoom

- (BOOL)validateMenuItem:(NSMenuItem *)item 
	{
	int itemTag = [item tag];
	if (((itemTag == 1) && ([windowModeWindow isVisible])) || ((itemTag == 1) && ([sheet isVisible])))  //WORKS
		return NO;
	if	(((itemTag == 2 || itemTag == 5 || itemTag == 6) && ([fullScreenModeWindow isVisible])) //WORKS
	        || ((itemTag == 2 || itemTag == 5 || itemTag == 6) && ([sheet isVisible])))  //DOESN"T WORK
		return NO;
	if ((itemTag == 4) && ([registrationSheet isVisible])) //WORKS
		return NO;
	return YES;
	}

any thoughts?