An error occurs when insert in the head tag

[domxdom]

Describe the bug
When using webpack to insert javascript into the head tag, I get the error focus-within-polyfill.js: 1 Uncaught TypeError: Cannot read property'getAttribute' of null

Expected behavior
DOMContentLoaded is required before executing addClass
https://github.com/matteobad/focus-within-polyfill/blob/master/src/focus-within.js#L121