In IE8, javascript fails when setting focus on intro HTML editor

1. Use IE8
2. Create a new File resource
3. Set a name, and add a file but leave description empty
4. Click on 'Save and return to course'

Expected behaviour:

• The HTML editor gets focus and you stay on the same page

What happens

• Javascript fails causing the form to be submitted

Can't move the focus to the control because it is invisible, not enabled, or of a type that does not accept the focus.

frm.elements['introeditor[text]'].focus();