Check whether creating a React Portal into a null container is safe on unmount and doesn't leak memory

[shirakaba]

We'll be using this trick a lot, as it's the only way to navigate.

May require special handling in the Host Config (e.g. to ensure that null.removeChild() is never called on Portal unmount). May also need to learn how properly to handle unmounting such a portal.

[shirakaba]

Now that I understand a bit more about detached trees, I believe that this won't be a problem in practice.