About "PopUpManager.createPopUp"

Hi,i get the window name from a xml file,for example:

function CreateTreePopuWin(ObjBtnSourceXml){
import mx.containers.TitleWindow;
import mx.managers.PopUpManager;
oInitObj.mainApp = this;
windowOBJ = eval(ObjBtnSourceXml.window);
var DetailsDialogInstance bject = PopUpManager.createPopUp(_root,windowOBJ, true, oInitObj, false);
}

But this has some matter ,because the "windowOBJ" has not instantiation.
How to do this?

thanks for your help!