问beginSheetModalForWindow方法错误

EN

@interface MyAppDelegate : NSObject <NSApplicationDelegate> {
    NSWindow *window;
}

@property (assign) IBOutlet NSWindow *window;

@end

@synthesize window;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{

}

@end

NSAlert *alert = [[[NSAlert alloc] init] autorelease];
        [alert setMessageText:@"Alert."];

        [alert beginSheetModalForWindow:window
                          modalDelegate:self
                         didEndSelector:@selector(alertDidEnd:returnCode:contextInfo:)
                            contextInfo:nil];