Runs when the picker is expanded.  Selects the appropriate tree node based on the value of the input element,
and focuses the picker so that keyboard navigation will work.
@private
function() {
        var me = this,
            picker = me.picker,
            store = picker.store,
            value = me.value,
            node;

        
        if (value) {
            node = store.getNodeById(value);
        }
        
        if (!node) {
            node = store.getRootNode();
        }
        
        picker.selectPath(node.getPath());

        Ext.defer(function() {
            picker.getView().focus();
        }, 1);
    }