关于使用模态视图时出现警告和报错的问题

1.A视图控制器：

BViewController *BVC = [[BViewController alloc]init];
[self presentViewController:BVC animated:YES completion:nil];

2.B视图控制器：

运行APP，点击左上角的返回按钮时，调用 

[self dismissViewControllerAnimated:YES completion:nil];

控制台提示警告：

Warning: Attempt to present <ModalViewController> on <ViewController>while a presentation is in progress

3.我修改了A视图控制器:

if(_BVC == nil){
    _BVC = [[BViewController alloc]init];
}
[self presentViewController:BVC animated:YES completion:nil];

运行APP，点击B视图控制器左上角的返回按钮，程序报错：

exception.reason: Application tried to present modally an active controller

4.在百度找到一贴，修改A视图控制器：

if(_BVC == nil){
    _BVC = [[BViewController alloc]init];
}

if (self.presentedViewController == nil) {
    [self presentViewController:BVC animated:YES completion:nil];
}