Question: I have encountered this warning:

> 
> pushViewController:animated: called on while an existing transition or presentation is
>  occurring; the navigation stack will not be updated.
> 
> 
> 

While trying to call `navigationController?.popViewControllerAnimated(false)` from `UIAlertController` completion block.

Answer: 
This warning indicates that you are trying use `UINavigationController` wrongly:

> 
> pushViewController:animated: called on while an existing transition or presentation is occurring; the navigation stack will not be updated
> 
> 
> 

You mentioned in the comments that you are trying to `pop` the
`ViewController` using 

```
navigationController?.popViewControllerAnimated(false)

```

inside completion block of `UIAlertController`. Therefore, you are trying to unwind from the wrong view, **`UIAlertController` is not part of the `UINavigationController` stack**.

Try and close the `UIAlertController` first, then pop the current `ViewController`. In other words, remove the `pop` from the `completion` block and put it inside the `OK` block. or use `unwind` segue before the alert.

Another possibility, is that you have an unused or identical duplicate in `storyboard`. Hence, if the `unwinding` operation is triggered by `storyboard` button, select this button and check the `connectivity inspector` and removed unwanted connections. 

For example: the red x marked was unnecessary in my case.
[![Example](https://i.stack.imgur.com/nZ281.png)](https://i.stack.imgur.com/nZ281.png)
Anyone needs go back to **PreviousVC** Use this code. **Swift5**

```
    dismiss(animated: true) {
        // If you need call someFunction()
        self.someFunction()
        self.navigationController?.popViewController(animated: true)
    }

```