从主视图中删除UIAlertview

应用显示UIAlertview，我使用NSTimer 调用removeFromSuperView删除警报。警报移除后，所有的操作都在当前视图进行了，应该怎么解决？
显示警告的代码：

UIAlertView *alert=[UIAlertView alloc]initWithTitle:@"A" message:@"B" delegate:self cancelButtonTitle:nil otherButtonTitles:nil, nil];
        [alert show];
        [alert release];
[NSTimer scheduledTimerWithTimeInterval:4.0 target:self selector:@selector(removeAlert:) userInfo:nil repeats:NO];

用timer调用方法:

-(void)removeAlert:(id)sender
{
    [alert_View removeFromSuperview];
}

取消UIAlertView的显示

[alert dismissWithClickedButtonIndex:0 animated:YES];