Problems with back-to-top.min.js

Hello!

After upgrading to V2 of GeneratePress I got errors from back-to-top.min.js on several pages. They only occur in combination with other plugins. I did not have the problems with these plugins before V2 so it could be a theme's problem.

For an example, you can check https://www.feelspace.de . There I activated WP Popup Maker and then two things happen:
1) A lot of JS errors show up:

back-to-top.min.js?ver=2.0:1 Uncaught TypeError: Cannot read property 'addEventListener' of null
    at back-to-top.min.js?ver=2.0:1
    at back-to-top.min.js?ver=2.0:1
(anonymous) @ back-to-top.min.js?ver=2.0:1
(anonymous) @ back-to-top.min.js?ver=2.0:1
back-to-top.min.js?ver=2.0:1 Uncaught TypeError: Cannot read property 'getAttribute' of null
    at back-to-top.min.js?ver=2.0:1
back-to-top.min.js?ver=2.0:1 Uncaught TypeError: Cannot read property 'getAttribute' of null
    at back-to-top.min.js?ver=2.0:1

2) My Social Icons from Lightweight Social Icons disappear and change into a white line. You can see them at the bottom of the page in the footer.

Do you have any ideas why these problems show up?

Thanks for any help!

Regards,
Heiko