MainActivity.kt

if (reviewInfo != null) {
       val reviewflow = reviewManager!!.launchReviewFlow(this, reviewInfo!!)
       reviewflow.addOnCompleteListener {
           Toast.makeText(
               applicationContext,
               "In App Rating complete",
               Toast.LENGTH_LONG
           ).show()
       }
   } else {
       Toast.makeText(applicationContext, "In App Rating failed", Toast.LENGTH_LONG).show()
   }
}