Check if _notification is still existent, before removing style-class in disable(), avoid possible error.

src/extension.js

@@ -355,6 +355,7 @@ function enable() {
  */
 function disable() {
     // remove our style, in case we just show a notification, otherwise the radius is drawn incorrect
+    if(Main.messageTray._notification)
         Main.messageTray._notification._table.remove_style_class_name('jrlnotification');
     Main.messageTray._showNotification = originalShowNotification;
     Main.messageTray._hideNotification = originalHideNotification;