Kotlin Android MainApplication class for global applicationContext.

KotlinAndroidMainApplication1.kt

// Author Keisuke Miura 
// Not object class. AndroidManifest.xml error happen.
class MainApplication : Application() {

    init {
        instance = this
    }

    companion object {
        private var instance: MainApplication? = null

        fun applicationContext() : Context {
            return instance!!.applicationContext
        }
    }

    override fun onCreate() {
        super.onCreate()
        // initialize for any
        
        // Use ApplicationContext.
        // example: SharedPreferences etc...
        val context: Context = MainApplication.applicationContext()
    }
}

KotlinAndroidMainApplication2.kt

// author Svetlozar Kostadinov (https://gist.github.com/sevar83)
class MyApp : Application() {

    init { INSTANCE = this }

    companion object {
        lateinit var INSTANCE: MyApp
            private set

        val applicationContext: Context get() { return INSTANCE.applicationContext }
    }
}

More:
https://blog.mindorks.com/how-to-create-a-singleton-class-in-kotlin