Update base.js

Problem occured during my dev

Resources/public/base.js

@@ -1,6 +1,6 @@
 jQuery(document).ready(function() {
     jQuery('html').removeClass('no-js');
-    if (window.SONATA_CONFIG.CONFIRM_EXIT) {
+    if (window.SONATA_CONFIG && window.SONATA_CONFIG.CONFIRM_EXIT) {
         jQuery('.sonata-ba-form form').confirmExit();
     }