When I close a window opened with window.open() I get an error from Firefox

Question

I am getting following error in firefox error console...

Error: uncaught exception: [Exception... "Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIDOMWindowInternal.alert]" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: http://www.aboriginalsystems.com/test/popup.html :: anonymous :: line 7" data: no]

I have an iframe in a page. In that page I have a link named "Do It". When I click on that link, an overlay on parent page will come and a popup will be opened. But when I close that popup it's giving me above error. I have called a function on "onunload" event of that popup.

Here is the link. Running example click me

What can be the cause?