After setting backIndicatorImage, how to reuse the system's back icon?

I wrote the following code and set a custom back button icon:

let navigationBar = UINavigationBar.appearance()

navigationBar.backIndicatorImage = image
navigationBar.backIndicatorTransitionMaskImage = image

It works, which is good.

But now in another controller, I want to reuse the system’s back button icon.

I have tried the following in the new controller:

1. Set backIndicatorImage and backIndicatorTransitionMaskImage to nil.
2. navigationItem.backBarButtonItem = nil

But this doesn't work.

My application needs to support iOS 10, so I cannot use SF Symbol.

What should I do?