How to create a tab bar controller with presents a nil modal view

i am creating an application which make uses of tabbar.i have created a function named setuptabbar which calls all the classes that use tab bar . this my code.

-(void)setupTabbar{
    //this is used to allocate space for tab bar item.
    TJourneylistController *journeylist =[[TJourneylistController alloc]initWithNibName:nil bundle:nil];
    UINavigationController *journeylistnavigation =[[UINavigationController alloc]initWithRootViewController:journeylist];
    [journeylist.tabBarItem initWithTitle:@"Journey List" image:[UIImage imageNamed:@""] tag:2];
    [journeylist release];

    TAppStoreController *appstore =[[TAppStoreController alloc]initWithNibName:nil bundle:nil];
    UINavigationController *appstorenavigation =[[UINavigationController alloc]initWithRootViewController:appstore];
    [appstore.tabBarItem initWithTitle:@"App Stroe" image:[UIImage imageNamed:@""] tag:1];
    [appstore release];

    TSettingsController *settings =[[TSettingsController  alloc]initWithNibName:nil bundle:nil];
    UINavigationController *settingsnavigation =[[UINavigationController alloc]initWithRootViewController:settings];
    [settings.tabBarItem initWithTitle:@"Journey List" image:[UIImage imageNamed:@""] tag:3];
    [settings release];

    TAboutController *about =[[TAboutController alloc]initWithNibName:nil bundle:nil];
    UINavigationController *aboutnavigation =[[UINavigationController alloc]initWithRootViewController:about];
    [about.tabBarItem initWithTitle:@"Journey List" image:[UIImage imageNamed:@""] tag:4];
    [about release];

    mTabController.viewControllers = [[NSArray alloc] initWithObjects:appstorenavigation,journeylistnavigation,settingsnavigation,aboutnavigation,nil];
    [appstorenavigation release];
    [journeylistnavigation release];
    [settingsnavigation release];
    [aboutnavigation release];
}


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after application launch.

    mTabController = [[UITabBarController alloc]init];
    [self setupTabbar];
    if([launchOptions objectForKey:UIApplicationLaunchOptionsRemoteNotificationKey]!=nil)
        mTabController.selectedViewController = [mTabController.viewControllers objectAtIndex:0];
    [mTabController presentModalViewController:mViewController animated:YES];
     //Screen *screen =[[Screen alloc]initWithNibName:@"Scr开发者_开发知识库een" bundle:nil];
    [window addSubview:mTabController.view];
    [self.window makeKeyAndVisible];
    //[screen release];

    return YES;
}

But when i run the project it gives me an error that 'Application tried to present a nil modal view controller.What is the problem.

In you following line:

[mTabController presentModalViewController:mViewController animated:YES];

You are presenting mViewController from mTabController. What is mViewController ? It's maybe because mViewController is never allocated. If you remove this line I think your code is working.