问题描述：

在ViewController中
当我在弹出UIAlertController的弹出框时，同时调用了 [self.navigationController popViewControllerAnimated:YES];这个方法
导致

popViewControllerAnimated: called on <UINavigationController 0x10201e600> while an existing transition or presentation is occurring; the navigation stack will not be updated.

分析原因

iOS8.0以后 UIAlertController弹出的动画效果，跟popViewController的动画效果冲突，导致无法正常Pop

解决方案

1. UIAlertController弹出以后0.5秒Pop

DispatchQueue.main.asyncAfter(deadline: 0.5) {
  self.navigationController?.popViewController(animated: true)
}

2. UIAlertController消失以后在Pop