Closing a dialog quickly causes a javascript error

When you open a dialog window in the cms or console and then close it quickly enough the following javascript error is logged to the javascript console:

this.document is null

This is in the disableFocus function at line 1405 in modal.js

Because this is code copied from wicket and adjusted to work not with iframes but inline elements, it could be that this piece of code is not needed anymore. In fact it should be looked if not all iframe related code should be removed from modal.js