UIAcExtVIEW退出Excel BADIX访问错误

我有这样的错误:当我单击navigationbar.backItemButton时,我显示的是带有两个按钮的UIAlertView。当我按下其中任何一个按钮时,应用程序将以EXC_BAD_访问权限终止。方法-(void)alertView:(UIAlertView*)alertView clickedbuttonindex:(NSInteger)buttonIndex不调用。我该怎么解决?谢谢!

//h-文件

@interface DetailsTableViewController : UITableViewController <UITextFieldDelegate, UIAlertViewDelegate>

//m-文件

- (void)viewWillDisappear:(BOOL)animated
{
    //if changes unsaved - alert reask window
    if (isDirty)
    {
        UIAlertView *message = [[UIAlertView alloc] initWithTitle:@"Save changes?"  
                                                      message:@"Press YES if you want to save changes before exit, NO - other case."  
                                                     delegate: self  
                                                    cancelButtonTitle: @"NO"  
                                                    otherButtonTitles: @"YES", nil];   

        [message show];  

        [message autorelease];
    }   
}

    - (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    NSString *title = [alertView buttonTitleAtIndex: buttonIndex];  

    if([title isEqualToString: @"YES"])  
    {  
        [self saveBtnUserClick];    
    } 
}