With 1.5 and prior the “back-to-top” icon worked fine.
Now after updating to 1.5.2 it doesnt work anymore.
There seems to be a problem of addressing the ID, which is not on the icon, but not on the a link.
If I remove the link from the icon, the back-to-top" is shown, but then FA Icon is not shown.
In your case, you can change the QuerySelector to .wd-back-to-top-icon, so that your custom JS works again. I’ll mark close this thread and mark it as duplicate.