I am not use null safety at now, how can I fix this problem on my build.gradle. I am trying to build my apk to appbuild and I am beginner.am I open the null safety or am I have a chance to do it without null safety. Thanks stackoverflow users. <3

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


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

android {
    compileSdkVersion 31

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

    lintOptions {
        disable 'InvalidPackage'
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.saka.bluetooth_controller"
        minSdkVersion 18
        targetSdkVersion 28
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }

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

flutter {
    source '../..'
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}