That's the beauty of open-source software ... you can find the error and fix it yourself.
In this case, the problem is in the org.jboss.portal.core.portlet.management.actions.AddWindowAction file's validateWindowName(FacesContext, UIComponent, Object) method. This method checks for empty strings and duplicate names, but doesn't check for null Window names first. It seems to be called for all the actions on the page too (I don't think that it needs to be called when moving a window, only creating a new one). Since this action is a Bean managed by MyFaces, it needs to be modified to simply return if a MOVE action is used.
There is a simple work-around for this problem. Type a few non-space characters into the WindowName field before clicking the arrow buttons.
I tried to add a note to the JIRA issue, but it claims I'm not logged in and don't have an account ... when I click on the link to create an account, it says I'm already logged in and (since I'm posting here in forums), it should be obvious that I have an account. Does anyone know how I can fix my account to log into JIRA.
I'd send you a patch, but I broke my Portal build environment and have been concentrating on creating Portlets. I'm using Portal-2.2.1-RC3 with this work-around.