add an extra check to ensure the pips exist before adding a class to them

src/Umbraco.Web.UI.Client/src/common/directives/components/umbrangeslider.directive.js

@@ -332,7 +332,10 @@ For extra details about options and events take a look here: https://refreshless
               activePip[handle] = pip;
             }
           });
-          activePip[handle].classList.add("noUi-value-active");
+
+          if (activePip[handle]) {
+            activePip[handle].classList.add("noUi-value-active");
+          }
         });
       }
       function addPipClickHandler(){

src/Umbraco.Web.UI.Client/src/installer/steps/user.controller.js

@@ -64,7 +64,10 @@ angular.module("umbraco.install").controller("Umbraco.Install.UserController", f
             activePip[handle] = pip;
           }
         });
-        activePip[handle].classList.add("noUi-value-active");
+
+        if (activePip[handle]) {
+          activePip[handle].classList.add("noUi-value-active");
+        }
       });
 
       $(consentSlider).on('$destroy', function () {