Program crashes after popToRootViewControllerAnimated

Question

I have a navigationController in MainMenuViewController class. When I push FirstViewController in navigationController I go to my second scene (FirstViewController) and it's ok. But when I'd want go back to root controller (MainMenuViewController) my program crash in main.m with error Thread 1:Program received signal: "EXC_BAD_ACCESS". Can you help me?

int main(int argc, char *argv[]) {

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    int retVal = UIApplicationMain(argc, argv, nil, nil);  // CRASH
    [pool release]; 
    return retVal;
}

ProjectAppDelegate.h:

@interface ProjectAppDelegate: NSObject <UIApplicationDelegate> {
    UIWindow *window;
    MainMenuViewController *mainVC;
    UINavigationController *navigationController;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;
@property (nonatomic, retain) IBOutlet MainMenuViewController *mainVC;
@property (nonatomic, retain) UINavigationController *navigationController;

+(ProjectAppDelegate.h*)getInstance;
@end

ProjectAppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
    ProjectAppDelegateInstance = self;
    self.navigationController = [[UINavigationController alloc] initWithRootViewController:mainVC];
    [self.navigationController setNavigationBarHidden:TRUE];
    [self.window addSubview:self.navigationController.view];
    [self.window makeKeyAndVisible];
    return YES;
}

MainMenuViewController.m

- (IBAction)actonFirst:(id)sender 
{
    FirstViewController *firstVC = [[[FirstViewController alloc] initWithPageNum:1] autorelease];
    [[ProjectAppDelegate getInstance].mainVC.navigationController pushViewController:firstVC animated:TRUE];
}

FirstViewController.m

- (IBAction)actonHome:(id)sender 
{
    [[ProjectAppDelegate getInstance].mainVC.navigationController popToRootViewControllerAnimated:TRUE];
}