dialog vs palette

How do I change a text in Illustrator using a window type PALETTE when using DIALOG PALETTE works but with no, someone can explain me why?

my script is...

#target Illustrator

var sourceDoc=app.activeDocument;

var myMaterials = ["FILME POLIÉSTER HOLOGRÁFICO", "FILME BOPP METALIZADO","FILME BOPP BRANCO","PAPEL COUCHÊ", "PVC PLANO"];

var myWin = new Window ("dialog","Materiais");

myWin.orientation = "row";

myNewList=myWin.add ("dropdownlist", undefined, myMaterials);

myNewList.selection = 0;

myOkButton=myWin.add ("button", undefined, "Ok",{name: "ok"});

myCancelButton=myWin.add ("button", undefined, "Cancelar",{name: "cancel"});

myNewList.onChange = function () {

    var item=myNewList.selection

    var myTextFrame = sourceDoc.textFrames.getByName("MATERIAL");

    myTextFrame.contents=item;

      }

myWin.show ();