I'm using Ext 4.0.2, and trying to 'fix' the back button for an Ajax-loaded page using Ext.util.History,
But when I put the following code on my page:

Code:
Ext.require(['Ext.util.History']);
Ext.util.History.init(  function(){
    Ext.util.History.on('change', function(token)
        {
        }
    );
});
I get:
Any advice on what I'm doing wrong so far?