I try to create a list of checkboxes which are filled dynamicaly by ExtDirect. I can see items in the dom but items are not displayed. What can i do?

Example:

Code:
var checkGroup = {
        xtype: 'fieldset',
        title: 'Group',
        autoHeight: true,
        layout: 'form',   
        items: [{
            xtype: 'checkboxgroup',
            fieldLabel: '',
            id:'idcheck'
           ,items: [{boxLabel: 'test', name: 'cb-auto-2'}]          
        }]
    };
    
    var myStore = new Ext.data.DirectStore({
            autoLoad: true,            
            directFn: MyNamespace.MyPHPClass.getListOfCheckbox,
            paramsAsHash: false,
            idProperty: 'id',
            listeners: {
                load: function(s, records){
                    var columns = Ext.getCmp('idcheck').items;
                    columns.clear();
                    i=0;
                    Ext.each(records, function(rec)
                    {
                     columns.add({boxLabel: rec.data.Libelle, name: 'mycheckname' + rec.data.id});
                     //columns.doLayout(); //doesnt work
                     i++;                     
                    }            
                     
                   );
                    
                }},            
            fields : 
                        [{
                        name : 'id',
                        type : 'int',
                        mapping : 'id'
                    },{
                        name : 'Libelle',
                        type : 'string',
                        mapping : 'Libelle'
                    }]
        });