Back To Top functionality doesn't work with Scrollbars

Hello! There is a bug or configuration conflict. My website theme has Back To Top button. I’m using Gantry 5 Framework based on Grav CMS. Here is a demo, how the Back to top function is made:

The problem is that on my website Back To Top doesn’t work once OverlayScrollbars is loaded. Clicking on anchor makes nothing and there are no errors in the console.

I’m initializing scrollbars like this:

$(function() {	
	$("body").overlayScrollbars({
          className: "os-theme-light custom-scroll",
        });
});

[KingSora]

function backToTopBtnClick() {
  var osInstance = $('body').overlayScrollbars();
  if (OverlayScrollbars.valid(osInstance)) {
    osInstance.scroll({ top: 0 }, 1000);
  }
  else {
    window.scrollTo({ top: 0, behavior: 'smooth' });
  }
}

[01Kuzma]

@KingSora thank you! I’ve added the onclick="backToTopBtnClick();" event to a link, everything works just fine 👍