Program received signal SIGABRT error

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WCF

REST

I am developing an iPhone app in which I have to call a REST serice from iPhone.I did it successfully and got the respons successfully.But later I have changed the xib file so that I want
to display the output on a label.Then after changing the xib file when I run the application I m
getting exception :

"Program received signal SIGABRT"

in the following code?Since Im a beginner to
iPhone developmet I dont know how to fix this error..

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary    
  *)launchOptions
 {
// Override point for customization after application launch.

self.window.rootViewController = self.viewController;    ///error at this line///
[self.window makeKeyAndVisible];
return YES;
}

How to fix this error?

Thanks...