Firebase FCM push, getting Error Missing Registration | Android
<p>I have seen this topic discussed before, but I think it's not the same scenario.</p>

<p>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.</p>

<p>I have tried to subscribe to topics or keep it simple still getting the same <strong>error</strong>: </p>

<blockquote>
  <p>MissingRegistration</p>
</blockquote>

<pre><code> 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;
                }
</code></pre>