Modal window does not open while adding image

In the create topic screen, when I click on image or link button, I see the form opening up abruptly in the form instead of in a modal window. Attached is the screenshot:





In js console, I see the following error when the create topic page loads:

TypeError: document.postform.subject.focus is not a function
www.jv-extensions.com/support-forum/jv-l...changer/topic/create
Line 1191

Can you please help.

Thanks
Niranjan