Flutter Could not find method android() for arguments() Error

Question

Out of nowhere I got an error. I do not know the reason. I would be glad if you help. I am getting the following error while running my project:

Launching lib\main.dart on Android SDK built for x86 in debug mode...

FAILURE: Build failed with an exception.

* Where:
Build file 'C:\Dosyalar\Yaz�l�m\Flutter\Simto-To-Do-List\simto_todolist\android\app\build.gradle' line: 21

* What went wrong:
A problem occurred evaluating project ':app'.
> Could not find method android() for arguments [build_6lhgp6v4laj4ufw7g9n0blrlp$_run_closure2@da9df8d] on project ':app' of type org.gradle.api.Project.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BU�LD FAILED in 1s
Exception: Gradle task assembleDebug failed with exit code 1
Exited (sigterm)

android\app\build.gradle:

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader ->
        localProperties.load(reader)
    }
}
def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}
def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}
def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

android {
    compileSdkVersion flutter.compileSdkVersion
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = '1.8'
    }
    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }
    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "io.simto.todolist.app"
        minSdkVersion 23
        targetSdkVersion flutter.targetSdkVersion
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
        multiDexEnabled true
    }
    buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig signingConfigs.debug
        }
    }
}
flutter {
    source '../..'
}

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

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'com.google.firebase:firebase-analytics-ktx'
    implementation 'com.google.guava:listenablefuture:9999.0-empty-to-avoid-conflict-with-guava'
    implementation "androidx.multidex:multidex:2.0.1"
    implementation platform('com.google.firebase:firebase-bom:29.2.1')
}

I got this error out of the blue and I don't know why. Also, this is the first time I'm getting such an error. How can I solve this problem? Thanks in advance for your help.