added null check

src/main/html/index.html

@@ -26,7 +26,7 @@
   <script type="text/javascript">
     $(function () {
       var url = window.location.search.match(/url=([^&]+)/);
-      if (url.length > 1) {
+      if (url && url.length > 1) {
         url = url[1];
       } else {
         url = "http://petstore.swagger.wordnik.com/v2/swagger.json";