[erp5_web_renderjs_ui] Do not ongoing changes if no action is available

bt5/erp5_web_renderjs_ui/PathTemplateItem/web_page_module/rjs_gadget_erp5_header_js.js

@@ -102,12 +102,16 @@
         // Directly modify the previous calculated header
         // in order not to remove the submit input
         // and still receive 'submit' event
-        var class_list = this.props.right_link.querySelector('button').classList;
+        var button = this.props.right_link.querySelector('button'),
+          class_list;
+        if (button !== null) {
+          class_list = button.classList;
           if (class_list.contains('ui-icon-check')) {
             class_list.remove('ui-icon-check');
             class_list.add('ui-icon-warning');
           }
         }
+      }
     })
     .declareMethod('notifySubmitting', function () {
       if (this.stats.submitted) {