How to make card layout visible?

Hello,

I've searched the web but I could not find a solution to this problem. In my Main.js I just use my own xtype "startseite", which uses "layout: 'card'". But for some reason the cards are invisible.

How to make them visible?

This is my code:

Code:

Ext.define('voc_trainer.view.Startseite', {
    extend: 'Ext.Panel',
    xtype: 'startseite',

    config: {
        layout: 'card',
        fullscreen: true,
        cls: 'startseite-cards',
        activeItem: 0,

        items: [
            {
                html: 'First Page'
            },
            {
                html: 'Second Page'
            }   
        ]
    }
});

How to make card layout visible?