Error: [core/not-initialized] Firebase has not been correctly initialized. Have you added the Firebase import scripts to your index.html file?

[khansawidhani]

Hi,
I am a flutter beginner and trying to connect my flutter app with firebase. I have followed the official documentation of Flutter Fire but I am failed to connect my app to firebase. Why I am seeing the error to import firebase in index.html if I am working on android app Have a look at the error:

Error: [core/not-initialized] Firebase has not been correctly initialized. Have you 
added the Firebase import scripts to your index.html file?
View the Web Installation documentation for more information:
https://firebase.flutter.dev/docs/installation/web
at Object.throw_ [as throw] (http://localhost:49800/dart_sdk.js:5348:11)
at
http://localhost:49800/packages/firebase_core_web/firebase_core_web.dart.lib.js:1    00:29
at initializeApp
(http://localhost:49800/packages/firebase_core_web/firebase_core_web.dart.lib.js:    106:29)
at initializeApp.next (<anonymous>)
at runBody (http://localhost:49800/dart_sdk.js:39250:34)
at Object._async [as async] (http://localhost:49800/dart_sdk.js:39281:7)
at firebase_core_web.FirebaseCoreWeb.new.initializeApp
(http://localhost:49800/packages/firebase_core_web/firebase_core_web.dart.lib.js:    85:20)
at initializeApp
(http://localhost:49800/packages/firebase_core/firebase_core.dart.lib.js:104:59) 
at initializeApp.next (<anonymous>)
at runBody (http://localhost:49800/dart_sdk.js:39250:34)
at Object._async [as async] (http://localhost:49800/dart_sdk.js:39281:7)
at Function.initializeApp
(http://localhost:49800/packages/firebase_core/firebase_core.dart.lib.js:103:20) 
at new main.MyApp.new
(http://localhost:49800/packages/restaurant_app/main.dart.lib.js:65:52)
at main$ (http://localhost:49800/packages/restaurant_app/main.dart.lib.js:82:20) 
at main (http://localhost:49800/web_entrypoint.dart.lib.js:36:29)
at main.next (<anonymous>)
at http://localhost:49800/dart_sdk.js:39230:33
at _RootZone.runUnary (http://localhost:49800/dart_sdk.js:39087:58)
at _FutureListener.thenAwait.handleValue
(http://localhost:49800/dart_sdk.js:34073:29)
at handleValueCallback (http://localhost:49800/dart_sdk.js:34633:49)
at Function._propagateToListeners (http://localhost:49800/dart_sdk.js:34671:17)  
at _Future.new.[_completeWithValue] (http://localhost:49800/dart_sdk.js:34513:23)    at http://localhost:49800/dart_sdk.js:33724:46
at _RootZone.runUnary (http://localhost:49800/dart_sdk.js:39087:58)
at _FutureListener.then.handleValue (http://localhost:49800/dart_sdk.js:34073:29)    at handleValueCallback (http://localhost:49800/dart_sdk.js:34633:49)
at Function._propagateToListeners (http://localhost:49800/dart_sdk.js:34671:17)  
at _Future.new.[_completeWithValue] (http://localhost:49800/dart_sdk.js:34513:23)    at async._AsyncCallbackEntry.new.callback
(http://localhost:49800/dart_sdk.js:34536:35)
at Object._microtaskLoop (http://localhost:49800/dart_sdk.js:39374:13)
at _startMicrotaskLoop (http://localhost:49800/dart_sdk.js:39380:13)
at http://localhost:49800/dart_sdk.js:34887:9  

build/app/build.gradle file:

apply plugin: 'com.android.application'
 apply plugin: 'com.google.gms.google-services'
 apply plugin: 'kotlin-android' 
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"

defaultConfig {
    applicationId "com.example.restaurant_app"
    minSdkVersion 16
    targetSdkVersion 30
    versionCode flutterVersionCode.toInteger()
    versionName flutterVersionName
    multiDexEnabled true
}
dependencies {
   implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
   implementation 'com.android.support:multidex:1.0.3'
}

android/build.gradle file:

buildscript {
ext.kotlin_version = '1.3.50'
repositories {
    google()
    jcenter()
}

dependencies {
    classpath 'com.android.tools.build:gradle:4.1.0'
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    classpath 'com.google.gms:google-services:4.3.8'
   }
 }

Also, I have added google-services.json file into android/app directory

What I've tried:

I have created a new firebase project and tried to connect the new project to the flutter app.

I have created a new flutter project and tried to connect a new flutter project to firebase as there may be cache issues. But all went in vain.

If anyone can let me know why I am getting error about index.html and why they are recommending me to see web installation.

Thanks in advance for your help if anyone can.