core not-initialized Firebase has not been correctly initialized

Android apply plugin: 'com.google.gms.google-services' at the end of /android/app/build.gradle and make sure you have places the google-service.json in your app directory /android/app/google-service.json

Web (in index.html)

<script src="https://www.gstatic.com/firebasejs/7.20.0/firebase-app.js"></script>
<script src="https://www.gstatic.com/firebasejs/8.4.1/firebase-firestore.js"></script>
<script>
  var firebaseConfig = {
    apiKey: "",
    authDomain: "",
    projectId: "",
    storageBucket: "",
    messagingSenderId: "",
    appId: "",
    measurementId: ""
  };
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig);
  firebase.analytics();
</script>

then flutter clean and run your app.

For people who get this error on IOS, make sure you copied the GoogleService-info.plist file using XCode.

Simply drag and dropping the file in the finder doesn't work.