Problems using treestore in viewmodel

If I create a treepanel and bind a treestore from the viewmodel. I get the following error.

Uncaught TypeError: Cannot read property 'isUtilObservable' of null

Code:

        treeStore: {
            type: 'tree',
            proxy: {
                type: 'ajax',
                url: './json/tree.json',
                reader: {
                    type: 'json'
                }
            },
            fields: [
                {
                    name: 'text'
                }
            ]
        }

If I make a similar store outside of the viewmodel it works just fine.

Code:

    constructor: function(cfg) {
        var me = this;
        cfg = cfg || {};
        me.callParent([Ext.apply({
            storeId: 'MyTreeStore1',
            proxy: {
                type: 'ajax',
                url: './json/tree.json',
                reader: {
                    type: 'json'
                }
            },
            fields: [
                {
                    name: 'text'
                }
            ]
        }, cfg)]);

Any suggestions would be appreciated.



Problems using treestore in viewmodel