ScriptUI: Script works alone, but not when called from UI button

* Edited to make Issue Clearer *

I've created a tool in Extendscript for Adobe Illustrator that will align and proportionately resize a selected group with a selected object named "Guide". The below code is the script, which works when run on it's own:

function proofTool() {
    var items = selection;
    if ( items.length != 2 ) //makes sure that 2 items are selected
    {
        alert("Select the and group the artwork and select the guide to center it on before running this script.");
    }
    else
    {
        //assigns selected guide to guide and other selection to artwork
        var artwork = null;
        var guide = null;
        for ( i = 0; i != 2; ++i )
        {
            if ( items[ i ].name == "Guide" )
            {
                guide = items[ i ];
            }
            else
            {
                artwork = items[ i ];
            }
        }
        // makes sure that things are sleected and got assigned
        if ( ( null == artwork ) || ( null == guide ) )
        {
            alert("Select the and group the artwork and select the guide to center it on before running this script.");
        }
        else
        {
            //Resizes artwork proportionately to fit in Guide area
            if (artwork.width >= artwork.height) 
            {
                var scale = (artwork.width / guide.width);
            } 
            else 
            {
                var scale = (artwork.height / guide.height);
            }
            artwork.width /= scale;
            artwork.height /= scale;

            //centers artwork on center of selected guide
            var guideXPos = (guide.position[0]+guide.width/2);
            var guideYPos = (guide.position[1]-guide.height/2);
            artwork.position = [guideXPos-(artwork.width/2), guideYPos+(artwork.height/2)];
            redraw();
        }//Close of Position/re-size If/Else
    }//Close of item select
}//Close of proofTool function

proofTool();

However, I wanted to create a palette that can be used to run this script, so that the user doesn't have to access the script through the menu. But, when I use the below script to create a palette with a button that calls that function, it stops at the line "var items = selection;". It sometimes gives an error stating that there is no document, but usually just runs until reaching that line, then stops (I added some $.writeln lines to see what where it was stopping). I tried changing that line to "var items = app.activeDocument.selection;" but that gave me an error stating that "app" was undefined. Any thoughts?

var win = new Window ('palette', 'Proof Tool');
    var okButton = win.add ('button', undefined, 'OK');
    okButton.onClick = proofTool;

function proofTool() {
    $.writeln ('Check 01');
    var items = selection;
    $.writeln ('Check 01.1');
    if ( items.length != 2 ) //makes sure that 2 items are selected
    {
        $.writeln ('Check 02');
        alert("Select and group the artwork and select the guide to center it on before running this script.");
    }
    else
    {
        $.writeln ('Check 03');
        //assigns selected guide to guide and other selection to artwork
        var artwork = null;
        var guide = null;
        for ( i = 0; i != 2; ++i )
        {
            if ( items[ i ].name == "Guide" )
            {
                guide = items[ i ];
            }
            else
            {
                artwork = items[ i ];
            }
        }
        // makes sure that things are sleected and got assigned
        if ( ( null == artwork ) || ( null == guide ) )
        {
            $.writeln ('Check 04');
            alert("Select and group the artwork and select the guide to center it on before running this script.");
        }
        else
        {
            $.writeln ('Check 05');
            //Resizes artwork proportionately to fit in Guide area
            if (artwork.width >= artwork.height) 
            {
                var scale = (artwork.width / guide.width);
            } 
            else 
            {
                var scale = (artwork.height / guide.height);
            }
            artwork.width /= scale;
            artwork.height /= scale;

            //centers artwork on center of selected guide
            var guideXPos = (guide.position[0]+guide.width/2);
            var guideYPos = (guide.position[1]-guide.height/2);
            artwork.position = [guideXPos-(artwork.width/2), guideYPos+(artwork.height/2)];
            redraw();
        }//Close of Position/re-size If/Else
    }//Close of item select
}//Close of proofTool function

win.show();
$.writeln ('Check 06');

You will have to change your window from a palette to a dialog. Apparently Illustrators implementation can't access the document from the palette. See this Thread in the Adobe forums https://forums.adobe.com/thread/841889 If you really need a palette there seems to be a workaround using BridgeTalk to run the code. Which seems silly to me :-/