Fix the issue frame is not define.

Closes #3421023

js/back_to_top.js

@@ -4,6 +4,8 @@
  */
 (function ($, Drupal, once, drupalSettings) {
 
+  // Define the frame var.
+  var frame;
   var scrollTo = function (to, duration) {
     var element = document.scrollingElement || document.documentElement,
       start = element.scrollTop,
@@ -46,7 +48,9 @@
       $(once('backtotop', '#backtotop'), context).each(function () {
         $(this).click(function () {
           $("html, body").bind("scroll mousedown DOMMouseScroll mousewheel keyup", function () {
-            window.cancelAnimationFrame(frame);
+            if (typeof frame !== 'undefined') {
+              window.cancelAnimationFrame(frame);
+            }
           });
           scrollTo(0, 1200);
         });