I need to add in a page size text variable to my already working .jsx script

Scott, is there a reason why this script works fine in Mac OS "High Sierra", but slows down Indesign to a crawl in "Sierra"? When i open the script in the editor i get this error:

If all the pages are the same size then you can use the below, if not then I would not recommend using text variables

See The intro comments

// https://forums.adobe.com/thread/2049382
// Main script by Loic Aigon Add highly customized Text Variables to InDesign | Ozalto
// Additions (First Page Dimensions and variable update on script execution)
// and modifications (without permission, sorry Loic) by Trevor http://www.creative-scripts.com
#targetengine 'usernameVariable'

//This function adds static text variables in InDesign
//They won't be dynamically updated though.
//That's why we need the updating function below

function toTitleCase(s) { // By Trevor
    return s.toLowerCase().replace(/\b\w/g, function () {return arguments[0].toUpperCase()});
}

function myRound(thisNumber, digits) { // By Trevor
        digits = (digits) ? Math.pow(10, digits) : 1000;
        return Math.round(thisNumber * digits) / digits;
}

function addVariables(openEvent){
    if (!openEvent) return;
    var doc = (openEvent.constructor == Document) ? openEvent : openEvent.parent; // By Trevor
    while ( doc.constructor.name != "Document" )
    {
        if ( doc.constructor.name == "Application" ){ return; }

       doc = doc.parent;
    }

    //Adding User name to text variables
    createTextVariable(doc, "User name", $.getenv($.os[0] == "M" ? "USER" : "username") ); // changed by Trevor
    //Adding Computer name to text variables
    createTextVariable(doc, "HD Name", (Folder.system).parent.displayName );

    //Adding Computer name to text variables
    createTextVariable(doc, "Computer Name", getComputerName() );

    //Adding a link to my website
    createTextVariable(doc, "_About Loic Aigon", "Feel free to visit my website: http://www.loicaigon.com !" );

    //Adding a link to my website
    createTextVariable(doc, "_About Trevor", "Feel free to visit my website: http://www.creative-scripts.com !" ); // by Trevor

    //Adding First Page Dimensions
    createTextVariable(doc, "First Page Dimensions", getPageDimentions(doc.pages[0])); // by Trevor

}

addVariables(app.properties.activeDocument); // callback by Trevor

//Generic function to add static custom text variables
function createTextVariable(target, variableName, variableContents){

    var usernameVariable = target.textVariables.itemByName(variableName);
    if(!usernameVariable.isValid){
        usernameVariable = target.textVariables.add();
        usernameVariable.variableType = VariableTypes.CUSTOM_TEXT_TYPE;
        usernameVariable.name = variableName;
    }
    usernameVariable.variableOptions.contents = variableContents;
}


//Snippet for grabbing the  deep name of the computer
function getComputerName(){ // by Trevor
    return $.os[0] == "M" ? app.doScript("get computer name of (system info)", ScriptLanguage.APPLESCRIPT_LANGUAGE) : $.getenv("computername");
}

function getPageDimentions(page){ // by Trevor
    var width, height, bounds, units, decPlaces;
    // function presumes that normal pages
    // i.e. same vertical and horizontal and no page shape transformations
    decPlaces = 3;
    units = toTitleCase(page.parent.parent.viewPreferences.horizontalMeasurementUnits.toString()); // If you forgot to include the toTitleCase function then remove the method
    bounds = page.bounds;
    height = bounds[2] - bounds[0];
    height = myRound(height, decPlaces);
    width = bounds[3] - bounds[1];
    width = myRound(width, decPlaces);
    return width + " X " + height + " " + units
}

//Add listeners to update file when opened.
app.addEventListener('afterOpen', addVariables);
// You might like to add some or all of these to make the update better
app.addEventListener('beforeSave', addVariables); // by Trevor
app.addEventListener('beforeSaveAs', addVariables); // by Trevor
app.addEventListener('beforeSaveACopy', addVariables); // by Trevor
app.addEventListener('beforeExport', addVariables); // by Trevor
app.addEventListener('beforePrint', addVariables); // by Trevor
app.addEventListener('beforeDeactivate', addVariables); // by Trevor
app.addEventListener('afterActivate', addVariables); // by Trevor