Firebase FCM push, getting Error Missing Registration | Android

Question

I have seen this topic discussed before, but I think it's not the same scenario.

I'm trying to send push notification from one device (going to be an admin device) through FCM (Firebase Cloud Messaging) to all other devices, and i'm going exactly by their docs.

I have tried to subscribe to topics or keep it simple still getting the same error:

MissingRegistration

 String jsonData = "{\"to\":\"/topics/news\",\"notification\":{\"title\":\"title\",\"body\":\"text\" }}";
                byte[] postData = jsonData.getBytes(Charset.forName("UTF-8"));
                int postDataLength = postData.length;

                URL url = new URL("https://fcm.googleapis.com/fcm/send");
                HttpURLConnection con = (HttpURLConnection) url.openConnection();
                con.setDoInput(true);
                con.setDoOutput(true);
                con.setInstanceFollowRedirects(true);
                con.setRequestMethod("POST");

                con.setRequestProperty("Content-Type","application/json");
                con.setRequestProperty("Authorization","key=AIzaSyB70J***-z34q2_h*******qjZsM5zBIf8Y"); //I've added stars :-) 
                con.setRequestProperty("Content-Type","application/x-www-form-urlencoded");
                con.setRequestProperty("Content-Type","charset=UTF-8");
                con.setRequestProperty("Content-Length",Integer.toString(postDataLength));

                con.setUseCaches(false);

                DataOutputStream wr = new DataOutputStream(con.getOutputStream());
                wr.write(postData);

                InputStream inputStream= con.getInputStream();
                BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
                String line = null;
                String outPut = "";
                while (((line = reader.readLine()) != null)){
                    outPut += line;
                }

Answer 1

I was using/trying to send FCM Push notification using Postman web API Client app, to FCM URL: https://fcm.googleapis.com/fcm/send

And I had used a wrong Content-Type: application/x-www-form-urlencoded, So I changed it to

Content-Type:  application/json

This is basically required, as we are sending the Push Notification as a JSON Payload.

Cheers!