Get Vuex store reference in cypress with vue-cli

I'm following along this tutorial to get the vuex store in cypress while running e2e tests.

As I use vue-cli (@vue/cli 4.3.1), with the @vue/cli-plugin-e2e-cypress. As of this setup, I don't have an app.js, rather a main.js.

But if I put the needed code in the main.js, it does not work:

/**
 * Start APP
 */
const app = new Vue({
    router,
    render: h => h(App),
}).$mount('#app');

if (window.Cypress) {
    console.log('Running in Cypress');

    // only available during E2E tests
    window.app = app;
} else {
    console.log('NOT Running in Cypress');
}

If I run it from cypress, I doesn't log anything from main.js to the console.

In cypress, when I try to get the store

cy.window().its('app.$store')

I get the error:

Timed out retrying: cy.its() errored because the property: app does not exist on your subject.

How can I get it running with vue-cli?