Submenu positioning missing in 6.0.0

**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
Unstyled submenu no longer has left/right positioning and appears over the top of the current context menu

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your CodeSandbox (https://codesandbox.io/s/new) example below:**
Tried this but ran into X is not a function using the example on react-contextify's website
https://codesandbox.io/s/upbeat-hamilton-sn3g71

**What is the expected behavior?**
Unstyled submenu appears to the right when there is room for it to display, or on the left if there is no screen space on the right.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
React 16.14.0, Chrome, working in version 5.0.0 of react-contextify, broken in 6.0.0

Reproduce the issue with the linked CodeSandbox using React 16.14.0, then compare submenu behavior between react-contextify 5.0.0 and 6.0.0. Done means an unstyled submenu opens to the right when space is available and to the left when the right side lacks room, without covering the current context menu.

Written by the indexing model from the issue text.