Merge pull request #1888 from /issues/1812-adm-purchase-listener-on-unsubscribe

Retrieve current ADM PurchasingListener assuming it returns a nullable.

Author: brismithers

OneSignalSDK/onesignal/core/src/main/java/com/onesignal/core/internal/purchases/impl/TrackAmazonPurchase.kt

@@ -121,7 +121,7 @@ internal class TrackAmazonPurchase(
         if (!canTrack) return
         try {
             val curPurchasingListener =
-                listenerHandlerField!![listenerHandlerObject] as PurchasingListener
+                listenerHandlerField!!.get(listenerHandlerObject) as PurchasingListener?
             if (curPurchasingListener !== osPurchasingListener) {
                 osPurchasingListener!!.orgPurchasingListener = curPurchasingListener
                 setListener()