Autocomplete race condition makes page unresponsive

The autocomplete field can throw a javascript exception that makes the page unresponsive.

To reproduce, run attached quickstart and:
1. Click the text field and type "a"
2. Quickly click the "hide" button, and
3. Quickly click the text field again and type another "a", leaving the text field focused.
4. A couple of javascript exceptions will be thrown, where the first on seems to break the ajax functionality. Clicking "show" has no effect now.

Attaching quickstart and screenshot of the exceptions.