creating response handler for push prompting

Author: emawby

android/src/main/java/com/onesignal/flutter/OneSignalPlugin.java

@@ -279,8 +279,7 @@ private void userProvidedPrivacyConsent(MethodCall call, Result reply) {
 
   private void promptPermission(MethodCall call, Result result) {
     boolean fallback = call.argument("fallback");
-    OneSignal.promptForPushNotifications(fallback);
-    replySuccess(result, null);
+    OneSignal.promptForPushNotifications(fallback, new OSFlutterPromptForPushNotificationPermissionResponseHandler(messenger, channel, result, "promptPermission"));
   }
 
   private void getDeviceState(Result reply) {
@@ -568,6 +567,22 @@ public void onFailure(OneSignal.ExternalIdError error) {
     }
   }
 
+  static class OSFlutterPromptForPushNotificationPermissionResponseHandler extends OSFlutterHandler
+            implements OneSignal.PromptForPushNotificationPermissionResponseHandler {
+    OSFlutterPromptForPushNotificationPermissionResponseHandler(BinaryMessenger messenger, MethodChannel channel, Result res, String methodName) {
+      super(messenger, channel, res, methodName);
+    }
+      
+    @Override
+    public void response(boolean accepted) {
+      if (this.replySubmitted.getAndSet(true)) {
+        OneSignal.onesignalLog(OneSignal.LOG_LEVEL.DEBUG, "OneSignal " + methodName + " handler called twice, ignoring! response");
+        return;
+      }
+      replySuccess(result, accepted);
+    }
+  }
+
   static class OSFlutterPostNotificationHandler extends OSFlutterHandler
             implements OneSignal.PostNotificationResponseHandler {