An error with class context

classError.js

var ISliderDriver = new Interface({
    // The currently active slide
    activeSlideNumber: Number, 

    // Initialize the slider
    initialize: Function,

    // go to the next slide
    next: Function,

    // go to the previous slide
    previous: Function,

    // arrange the items
    arrange: Function,

    // rearrange the items
    rearrange: Function,

    // jump to a given slide
    jumpTo: function(slideNumber){},

});

var UniversalSlider = new Class({

    Implements: [Options],

    started: false,
    driverObject: null,

    options: {
        slides: [], // array of slides
        startIndex: 0, // where to start
        driver: 'Fade',
        driverPath: 'slider/driver/',
        autoStart: true,
        autoSlide: true,
        autoSlideInterval: 2000
    },

    initialize: function(slider, options){
        // convert the options to attributes
        this.setOptions(options);
        // set the slider object
        this.options.sliderObject = slider;
        // load the driver
        this.loadDriver(this.options.driver, this.options.driverPath);
        // arrange the items
        this.arrange();
        if(this.autoStart)
        {
            // set the slider started
            this.start();
        }

        // slide
        this.slide();
    },

    loadDriver: function(driver, driverPath){
        var path = driverPath+driver+'.js';
        var self = this;

        var driverFile = Asset.javascript(path , {
            onLoad: function(){
                try {
                    var driverObject = new window[driver](this.options)
                    self.driverObject = driverObject;
                    console.log(self.driverObject); // works here
                } catch (e){
                    if (this.console) this.console.error(e);
                    // load default driver here
                }
            }
        });

        console.log(self.driverObject); // doesn't work here
    },

    arrange: function(){
        
    },

    slide: function(){
        if(this.started){
            // the slider is started
        } else {
            // the slider is stopped
        }
    },

    start: function(){
        this.started = true;
    },

    stop: function(){
        this.started = false;
    },

});