ExtJS: render tooltip only when specific value is set

gistfile1.js

, listeners: {
    render: function(c) {
        // first get count via ajax call
        MODx.Ajax.request({
            url: className.config.connectorUrl,
            params: {
                action: 'mgr/group/getlist'
            },
            listeners: {
                'success': {
                    fn: function(r) {
                        var groupsCount = parseInt(r.total);
                        if (!groupsCount) {
                            Ext.getCmp('btn-add-file').disable();
                            Ext.QuickTips.register({
                                target: c,
                                text: _('classname.file.nogroups'),
                                showDelay: 40,
                                trackMouse: false
                            });
                        }

                    }
                }
            }
        });
    }
}