Android Firebase Notification Key - File Not Found Exception

Question

I tried to generate notification_key for firebase push notification in android using the following code from the firebase doc.

public String addNotificationKey(
    String senderId, String userEmail, String registrationId, String idToken)
    throws IOException, JSONException {
URL url = new URL("https://android.googleapis.com/gcm/googlenotification");
HttpURLConnection con = (HttpURLConnection) url.openConnection();
con.setDoOutput(true);

// HTTP request header
con.setRequestProperty("project_id", senderId);
con.setRequestProperty("Content-Type", "application/json");
con.setRequestProperty("Accept", "application/json");
con.setRequestMethod("POST");
con.connect();

// HTTP request
JSONObject data = new JSONObject();
data.put("operation", "add");
data.put("notification_key_name", userEmail);
data.put("registration_ids", new JSONArray(Arrays.asList(registrationId)));
data.put("id_token", idToken);

OutputStream os = con.getOutputStream();
os.write(data.toString().getBytes("UTF-8"));
os.close();

// Read the response into a string
InputStream is = con.getInputStream();
String responseString = new Scanner(is, "UTF-8").useDelimiter("\\A").next();
is.close();

// Parse the JSON string and return the notification key
JSONObject response = new JSONObject(responseString);
return response.getString("notification_key");}

But it returns file not found exception at InputStream is = con.getInputStream(); with response code 400.

Please help with the possible causes for this.