well, my understanding of your problem is as below.
You have say, parent.jsp, popup.jsp
Upon accessing "Add" button in your parent.jsp it opens popup.jsp (action 1)
Now on performing some selection in popup.jsp you submit the form (action 2) to add it in the parent jsp.
To solve your problem,
After performing "action 2" in your action class forward control back to "action 1" using struts (what it does is, it would refresh your popup, you would not get the blank screen). While doing so, just put up some param to identify that this is coming after performing "action 2".
Now when the popup reloads check for this param and refresh the parent window as mentioned earlier. Thats it!
In the above code you are forwarding it to dummy (which is not the actual popup.jsp) jsp which is not necessary and leading you to blank screen.
Additionally, If you do not want your pop up to be refreshed, just use asyn call (AJAX) to do it for you.