ios键盘在popViewControllerAnimated 时又弹起

//返回到上一界面
-(void)customBackBtnPressed:(id)send{

       UIAlertView *alertView=  [[UIAlertView alloc] initWithTitle:@"温馨提示" message:@"还差一步就完成注册！  确定放弃？"  delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定",nil];
        [alertView show];
    
}
//通常用的这个代理方法来做 发现键盘在popViewControllerAnimated后会重新出现
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{
    if (buttonIndex==1) {
        [self.navigationController popViewControllerAnimated:YES];
    }
}

//百思不得其姐后，后了下面这个代理方法，竟然键盘不出现了！
- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex
{
    if (buttonIndex==1) {
        [self.navigationController popViewControllerAnimated:YES];
    }
}