about deployment target in ios

i have created my app in ios 6 and now iset deployment target to 4.3 and if run on iphone 4.3 simulator my app crashes in appdelegate file it self. may i know possible reasons

     for(UIViewController *tab in  self.tabBarController.viewControllers)
         
     {
         [tab.tabBarItem setTitleTextAttributes:[NSDictionary dictionaryWithObjectsAndKeys://bad access (code=2)address=0)
                                                 [UIFont fontWithName:@"TimesNewRomanPS-BoldMT" size:16.0], UITextAttributeFont, nil]
                                       forState:UIControlStateNormal];
     }

i have this code in appdelegate. if i run on iphone 4.3 simulator .here my app crashes. iam not getting any idea help me thank u