I've enabled ARC, In my `didFinishLaunchingWithOptions` method, I wrote the following code:

AppDelegate.h:

```
@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (strong, nonatomic) ViewController *viewController;

@end

```

AppDelegate.m:

```
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    ViewController * vc = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    UINavigationController * nav = [[UINavigationController alloc] initWithRootViewController:vc];
    self.viewController = nav;
    self.window.rootViewController = self.viewController;
    [self.window makeKeyAndVisible];
    return YES;
}

```

But the statement: `self.viewController = nav;` gets a compile warning, the warning info is:

`file://.../AppDelegate.m: warning: Semantic Issue: Incompatible pointer types passing 'UINavigationController *__strong' to parameter of type 'ViewController *'`

![Compile Warning Information](https://i.stack.imgur.com/n1FnS.png)

How to remove the warning? 

Thanks.