LoginScreen.js

...

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

...

export default function LoginScreen({navigation}) {

    ...

    const onLoginPress = () => {
        firebase
            .auth()
            .signInWithEmailAndPassword(email, password)
            .then((response) => {
                const uid = response.user.uid
                const usersRef = firebase.firestore().collection('users')
                usersRef
                    .doc(uid)
                    .get()
                    .then(firestoreDocument => {
                        if (!firestoreDocument.exists) {
                            alert("User does not exist anymore.")
                            return;
                        }
                        const user = firestoreDocument.data()
                        navigation.navigate('Home', {user})
                    })
                    .catch(error => {
                        alert(error)
                    });
            })
            .catch(error => {
                alert(error)
            })
    }

    ...

}

@mrcflorian You're not supposed to manually navigate when conditionally rendering screens. See here: https://reactnavigation.org/docs/auth-flow/#dont-manually-navigate-when-conditionally-rendering-screens.

Suggested that this be removed: navigation.navigate('Home', {user}) (line 27)