Kotlin Unresolved Reference Error in Flutter MainActivity File

Discussion: Kotlin Unresolved Reference Error in Flutter MainActivity
This issue has been created to discuss the "Unresolved reference" errors in the MainActivity.kt file of a Flutter project.

Summary
I am facing an issue where my MainActivity.kt file shows "Unresolved reference" errors for FlutterActivity, FlutterEngine, and configureFlutterEngine. I have already opened the Android project in Android Studio (https://stackoverflow.com/questions/64276592/unresolved-reference-flutteractivity/64359540#64359540),
but the errors persist.

I’d like to discuss possible solutions for this issue and understand why it might be happening. Any help or suggestions would be appreciated! 🚀

------------flutter doctor------------
[√] Flutter (Channel stable, 3.29.0, on Microsoft Windows [Version 10.0.22631.4890], locale en-US) [471ms]
• Flutter version 3.29.0 on channel stable at C:\flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 35c388afb5 (2 weeks ago), 2025-02-10 12:48:41 -0800
• Engine revision f73bfc4522
• Dart version 3.7.0
• DevTools version 2.42.2

[√] Windows Version (11 Pro 64-bit, 23H2, 2009) [1,305ms]

[√] Android toolchain - develop for Android devices (Android SDK version 35.0.0) [2.8s]
• Android SDK at C:\Users\admin\AppData\Local\Android\sdk
• Platform android-35, build-tools 35.0.0
• Java binary at: C:\Program Files\Java\jdk-17\bin\java
This JDK is specified in your Flutter configuration.
To change the current JDK, run: flutter config --jdk-dir="path/to/jdk".
• Java version Java(TM) SE Runtime Environment (build 17.0.12+8-LTS-286)
• All Android licenses accepted.

[√] Chrome - develop for the web [188ms]
• Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[!] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.5.0) [187ms]
• Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
• Visual Studio Community 2022 version 17.5.33414.496
• Windows 10 SDK version 10.0.22000.0
X The current Visual Studio installation is incomplete.
Please use Visual Studio Installer to complete the installation or reinstall Visual Studio.

[√] Android Studio (version 2024.2) [25ms]
• Android Studio at C:\Program Files\Android\Android Studio1
• Flutter plugin can be installed from:
https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
https://plugins.jetbrains.com/plugin/6351-dart
• Java version openjdk version "21.0.4" 2024-07-16

[√] VS Code (version 1.97.2) [23ms]
• VS Code at C:\Users\admin\AppData\Local\Programs\Microsoft VS Code
• Flutter extension version 3.104.0

[√] Connected device (4 available) [364ms]
• SM A032F (mobile) • R9ZW406EW5M • android-arm • Android 13 (API 33)
• Windows (desktop) • windows • windows-x64 • Microsoft Windows [Version 10.0.22631.4890]
• Chrome (web) • chrome • web-javascript • Google Chrome 131.0.6778.71
• Edge (web) • edge • web-javascript • Microsoft Edge 133.0.3065.69

[√] Network resources [543ms]
• All expected network resources are available.

! Doctor found issues in 1 category.

------------ android/build.gradle ------------

buildscript {
ext.kotlin_version = '2.1.0'
repositories {
google()
mavenCentral()
}

dependencies {
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    classpath 'com.android.tools.build:gradle:8.7.2'
    classpath 'com.google.gms:google-services:4.3.15'
}

}

allprojects {
repositories {
google()
mavenCentral()
}
}

rootProject.buildDir = '../build'
subprojects {
project.buildDir = "${rootProject.buildDir}/${project.name}"
}
subprojects {
project.evaluationDependsOn(':app')
}

tasks.register("clean", Delete) {
delete rootProject.buildDir
}

------------android/app/build.gradle------------

plugins {
id "com.android.application"
id "kotlin-android"
id "dev.flutter.flutter-gradle-plugin"
id "com.google.gms.google-services"
}

// apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
localPropertiesFile.withReader('UTF-8') { reader ->
localProperties.load(reader)
}
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
flutterVersionName = '1.0'
}

def keystoreProperties = new Properties()
def keystorePropertiesFile = rootProject.file('key.properties')
if (keystorePropertiesFile.exists()) {
keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
}

android {
namespace "com.findmedicine"
compileSdkVersion 34
ndkVersion flutter.ndkVersion

compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
}  

kotlinOptions {
    jvmTarget = JavaVersion.VERSION_1_8
}

sourceSets {
    main.java.srcDirs += 'src/main/kotlin'
}

defaultConfig {
    applicationId "com.findmedicine"
    minSdkVersion 27
    targetSdkVersion 34
    versionCode flutterVersionCode.toInteger()
    versionName flutterVersionName
    multiDexEnabled true
    
}

signingConfigs {
    release {
        keyAlias keystoreProperties['keyAlias']
        keyPassword keystoreProperties['keyPassword']
        storeFile keystoreProperties['storeFile'] ? file(keystoreProperties['storeFile']) : null
        storePassword keystoreProperties['storePassword']
    }
}

buildTypes {
    release {
        signingConfig signingConfigs.release
    }
}

}

flutter {
source '../..'
}

dependencies {
implementation(platform("com.google.firebase:firebase-bom:32.7.4"))
implementation("com.google.firebase:firebase-analytics-ktx")
implementation 'androidx.multidex:multidex:2.0.1'
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
implementation 'androidx.core:core-ktx:1.6.0'
implementation 'androidx.appcompat:appcompat:1.3.1'
}

------------ gradle-wrapper.properties ------------
distributionBase=GRADLE_USER_HOME
distributionPath=wrapper/dists
zipStoreBase=GRADLE_USER_HOME
zipStorePath=wrapper/dists
distributionUrl=https://services.gradle.org/distributions/gradle-8.10-all.zip

------------error------------