Extending computed property of administration component

Shopware 6 (English) Programming (EN)
1

Hello,

I tried to extend a component in the administration, like explained on this page: https://docs.shopware.com/en/shopware-platform-dev-en/developer-guide/administration/inheritance

I want to add an extra menu option in the sidebar list on the Content -> Shopping Experiences page. So I created a plugin with the cli. In /src/Resources/app/administration/src/main I created the following code:

const { Component } = Shopware;

Component.extend('sw-cms-list', {
    computed: {
        sortPageTypes() {
            return [
                { value: '', name: this.$tc('sw-cms.sorting.labelSortByAllPages'), active: true },
                { value: 'page', name: this.$tc('sw-cms.sorting.labelSortByShopPages') },
                { value: 'landingpage', name: this.$tc('sw-cms.sorting.labelSortByLandingPages') },
                { value: 'product_list', name: this.$tc('sw-cms.sorting.labelSortByCategoryPages') },
                { value: 'custom_form', name: 'Custom form' }
            ];
        }
    }
});

My console shows an error: Uncaught TypeError: Cannot read property ‘template’ of undefined
    at Object.extend (commons.js?1588970146199238:1)
    at Object.0ZX1 (custom-forms.js?1588970147777:1)

So, what is exactly wrong with my extend? Why do I need to define a template since I only want to overwrite one function from computed?