conditional callback

app/assets/javascripts/app/services/apiController.js

@@ -219,7 +219,9 @@ angular.module('app.frontend')
       this.syncWithOptions = function(callback, options = {}) {
         if(!this.user.uuid) {
           this.writeItemsToLocalStorage();
-          callback();
+          if(callback) {
+            callback();
+          }
           return;
         }
 

vendor/assets/javascripts/transpiled.js

@@ -1750,7 +1750,9 @@ var User = function User(json_obj) {
 
       if (!this.user.uuid) {
         this.writeItemsToLocalStorage();
-        callback();
+        if (callback) {
+          callback();
+        }
         return;
       }