Create new Mail button without function

If I want to click the "create new Mail" button, nothing happens. 
But the console logs the following: 

> Uncaught TypeError: Cannot set property 'href' of null
>     at setFavicon (script.js:790)
>     at HTMLDocument.init (script.js:793)
> setFavicon @ script.js:790
> init @ script.js:793
> 

Unclear if those are related. 