关闭多个UIViewController时报异常，google了下，用下面代码可以避免，具体原因可能是关闭一个UIViewController影响到了其他的UIViewController，所以需要关闭了以后再关闭另一个，不能使用循环一下同时关闭多个，不知对不对，还请高手指教。上代码：
//这个是我定义的数组，放了多个需要关闭的UIViewController

@property(nonatomic, strong)NSMutableArray *viewControllers;

-(void)dismissViewControllerAll{
    if (_viewControllers != nil && _viewControllers.count>0) {
        UIViewController *controller = [_viewControllers lastObject];
        NSLog(@"dismissViewControllerAll controller=%@", controller);
        if (controller != nil) {
            [controller dismissViewControllerAnimated:NO completion:^{//不用此种方式，会异常：Trying to dismiss the presentation controller while transitioning already
                [_viewControllers removeObject:controller];
                [self dismissViewControllerAll];
            }];
        }else{
            [_viewControllers removeObject:controller];
            [self dismissViewControllerAll];
        }
    }
}