Showhide onclick event fails

When clicking on the dropdown button, the listener fails and the dropdown is not shown.
Tested on xwiki.org

Here is the error log I can get in the browser:

```
Uncaught ReferenceError: Effect is not defined
showhide1 https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/Tutorials/Skins/CSSLayout/:893
onclick https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/Tutorials/Skins/CSSLayout/:1
```