为什么我在API中获取变量值时出错？

Hi have this error when I try to send accents from android to an php api:

Notice: Trying to get property of non-object in /storage/ssd1/452/8869452/myapi.php

I use postman to test my API and I sent accenst with It and all works, but when I try to sent accents from android I get the error.

$respuesta["mensaje"] = array();
$json=file_get_contents('php://input');
$obj=json_decode($json);
$idresp=filter_var($obj->idresp);`

My json object that I sent It is the next:

{
    "QR":"76CD1L3hOb",
    "idresp":"53",
    "idportador":"4"
}

My android code it's the next

 JSONObject jsonUpdate = new JSONObject();
                    try {
                        jsonUpdate.put("QR", QR);
                        jsonUpdate.put("idresp", idusuario);
                        jsonUpdate.put("idportador",txtidPortador.getText().toString().trim());

                        new AgregarPortador(dialogoRecuperarCuenta).execute("myURLofMyApi.php", jsonUpdate.toString());
                    } catch (JSONException e) {
                        Log.e("Error :c", "El error:
" + e);
                    }//Cierra catch

The asynchTask code:

        @Override
        protected String doInBackground(String... strings) {
            String data = "";
            HttpURLConnection httpURLConnection = null;
            try {
                httpURLConnection = (HttpURLConnection) new URL(strings[0]).openConnection();
                httpURLConnection.setRequestMethod("POST");
                httpURLConnection.setDoOutput(true);
                DataOutputStream wr = new DataOutputStream(httpURLConnection.getOutputStream());
                wr.writeBytes(strings[1]);
                wr.flush();
                wr.close();
                InputStream in = httpURLConnection.getInputStream();
                InputStreamReader inputStreamReader = new InputStreamReader(in);
                int inputStreamData = inputStreamReader.read();
                while (inputStreamData != -1) {
                    char current = (char) inputStreamData;
                    inputStreamData = inputStreamReader.read();
                    data += current;
                }
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                if (httpURLConnection != null) {
                    httpURLConnection.disconnect();
                }
            }
            return data;
        }