App.js

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import { firebase } from './src/firebase/config'

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export default function App() {

  ...

  if (loading) {	
    return (	
      <></>	
    )	
  }

  useEffect(() => {
    const usersRef = firebase.firestore().collection('users');
    firebase.auth().onAuthStateChanged(user => {
      if (user) {
        usersRef
          .doc(user.uid)
          .get()
          .then((document) => {
            const userData = document.data()
            setLoading(false)
            setUser(userData)
          })
          .catch((error) => {
            setLoading(false)
          });
      } else {
        setLoading(false)
      }
    });
  }, []);

  ...

}