Uncaught TypeError: Cannot read property 'className'

Hello,
I get this error each time I clik in any menú button in the web I'm developing.
I see this in the error console.
This is the complet message I get:

Uncaught TypeError: Cannot read property 'className' of null
at HTMLAnchorElement.t (a11y.min.js?ver=2.2.2:1)
t @ a11y.min.js?ver=2.2.2:1

If I do hover over the "a11y.min.js?ver=2.2.2:1" this is the path error:
https://heremydomain.com/wp-content/themes/generatepress/js/a11y.min.js?ver=2.2.2

at same time, this web is breaking to many times under development (not a hosting issue because this is a dedicates server)