[[UIApplication sharedApplication].keyWindow.rootViewController presentViewController:alertVC animated:YES completion:nil];

😆因为在view上找不到控制器，而UIAlertController继承与controller，所以如果要在当前view推出，需要用到当前窗口的跟控制器。

另外，补充一句，也可以用UIActionSheet来推出弹窗：
UIActionSheet *sheet = [[UIActionSheet alloc] initWithTitle:@"" delegate:self cancelButtonTitle:@"取消" destructiveButtonTitle:nil otherButtonTitles:nil, nil];
[sheet showInView:self.view];