app.addEventListener is undefined

Hi

I'm building a palette via scriptingUI. I'm trying to create a palette that changes depending on what you have selected. Is this not possible in AE?

This is a test code just to trouble shoot:

function SelectionViewer()

{

    var selectionViewer = this;

    this.buildUI = function (thisObj)

    {

        // dockable panel or palette

        var pal = (thisObj instanceof Panel) ? thisObj : new Window("palette", "selectionViewer", undefined, {resizeable:false});

        var testText = pal.add("statictext", [2, 2, 201, 20], "Selected Item");

    }

    // Methods

    selectionViewer.updateSelected = function()

    {

        testText.text = app.project.activeItem;

    }

    this.run = function (thisObj)

    {

        this.buildUI(thisObj);

        app.addEventListener("afterSelectionChanged", selectionViewer.updateSelected);

    };

}

new SelectionViewer().run(this);

It gives me this error:

"Unable to execute script at line 24. Funtion app.addEventListener is undefined"

Any help?

Thanks,

Wagner