MainActivity.kt

private var backPressedOnce = false

override fun onBackPressed() 
{
    var navController = findNavController(R.id.fragNavHost)
    
    // Check if the current destination is actually the start sestination (Home screen)
    if (navController.graph.startDestination == navController.currentDestination?.id)
    {
        // Check if back is already pressed. If yes, then exit the app.
        if (backPressedOnce)
        {
            super.onBackPressed()
            return
        }

        backPressedOnce = true
        Toast.makeText(this, "Press BACK again to exit", Toast.LENGTH_SHORT).show()

        Handler().postDelayed(2000) {
            backPressedOnce = false
        }
    }
    else {
        super.onBackPressed()
    }
}