Black screen appears when dismissing controller to the root controller

## What did you do?

I tried to present a controller using over current context. After that used hero again to present another controller normally. (eg., project. attached where clicking on green view 2 times presents different controllers and click on 3rd green view dismisses the controller)

## What did you expect to happen?

All the controllers are dismissed showing main controller

## What happened instead?

After dismissing all the presented controllers, black screen appears.

## General Information

* Hero Version: 1.6.1

* iOS Version(s): 15.0 (Beta simulator)

* Swift Version: 5

* Devices/Simulators: iPhone

* Reproducible in Examples? (Yes/No): Yes

## Demo Project

[SampleHero.zip](https://github.com/HeroTransitions/Hero/files/7038746/SampleHero.zip)

Start by running the attached SampleHero.zip demo with Hero 1.6.1 on the specified iOS 15 simulator and reproduce the sequence of presenting controllers over the current context, presenting another controller, and dismissing them. Trace the dismissal flow until the black screen appears; done means the final dismissal returns to the main controller instead.

Written by the indexing model from the issue text.