There's no None it seems

templates/feedbackform.html

@@ -73,7 +73,7 @@
 <script type="text/javascript">
     var call = JSON.parse('{{ @call | raw }}');
     var call_url = call['data']['url'];
-    var interval = None;
+    var interval = '';
     function normalize_phone(phone) {
         if (phone.startsWith('+')) {
             return phone.replace('+', '00');