'NSObject' may not respond to -navigationController

@interface alerts : NSObject {

}

- (void)pleaseRegisterDevice;

@end


/////.m
#import "alerts.h"
#import "ViewController.h"
#import "AppDelegate.h"


@implementation alerts

//use this alert when phone falls out of sync
- (void)pleaseRegisterDevice {
   ViewController *viewController = [[ViewController alloc]initWithNibName:@"ViewController" bundle:nil];
            AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
            [appDelegate.navVC pushViewController:viewController animated:YES];
}

@end