ServerProxy URL

I am very confused by an error I am getting with the following XML store:

Code:

Ext.define('MyApp.store.UserStore2', {
    extend: 'Ext.data.Store',


    requires: [
        'MyApp.model.UserModel',
        'Ext.data.proxy.Ajax',
        'Ext.data.reader.Xml'
    ],


    constructor: function(cfg) {
        var me = this;
        cfg = cfg || {};
        me.callParent([Ext.apply({
            model: 'MyApp.model.UserModel',
            storeId: 'UserStore2',
            proxy: {
                type: 'ajax',
                url: '',
                reader: {
                    type: 'xml',
                    record: 'row'
                }
            }
        }, cfg)]);
    }
});

As you can see the proxy has no URL, this is because elsewhere in my app I send an AJAX call which returns an XML document that I place into the store by doing:

Code:

store.loadRawData(xmlDocument)

, so I shouldn't need a URL right?

I have associated this store with a grid panel and it works perfectly, the xml document gets loaded into the store and the records are displayed on the grid with no problems, but then I associate this very same store to a comboBox:

Code:

{
                            xtype: 'combobox',
                            itemId: 'categoryTypeField',
                            width: 405,
                            fieldLabel: 'Type',
                            displayField: 'username',
                            store: 'UserStore2',
                            valueField: 'username',
                            listeners: {
                                select: {
                                    fn: me.onCategoryTypeFieldSelect,
                                    scope: me
                                }
                            }
                        }

and as soon as I click on drop-down I get the following error: Ext.data.proxy.Server.buildUrl(): You are using a ServerProxy but have not supplied it with a url. and it gets stuck like this:

error.PNG

so clearly the records are getting loaded but then runs into this problem, I don't understand why it's giving me this error here but not with the grid panel, why does it require a URL all of a sudden? can someone please help me with this? it's halted progress with my entire app. If not then maybe some kind of workaround? all I need is to load records from the XML document I get back from the server so I can list them as value for this combo box, any help would be greatly appreciated.