When I am trying to access an element of my Parent window from a Pop-up window, I am getting window.opener
as undefined.
var opener = window.ope
I had the same problem and it was due to Internet Explorer Security Options, in particular because my popup was going to an External website (Internet area) and the parent was an internal page (Intranet area). The "Protected Mode" was only activated for the "Internet". I activated it for the "Local intranet" and now it works.
To find this option in IE: