Cannot read property 'then' of undefined... in index.js

[mohsinraza2223]

store.firebaseAuthIsReady.then(()=>{
ReactDOM.render(, document.getElementById('root'));

// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister();

})