[#2922677] Uncaught TypeError: Cannot read property 'replace' of undefined

Closes #2922677

core/misc/ajax.js

@@ -465,6 +465,7 @@
 
     // If no Ajax callback URL was given, use the link href or form action.
     if (!this.url) {
+      this.url = '';
       const $element = $(this.element);
       if (this.element.tagName === 'A') {
         this.url = $element.attr('href');