Error in Android connection with Web Service Restfull

I'm developing an android app for a course in college. I am connecting the Restfull Web Service with the android using the Gson library to pass data.

In the method below I am performing a POST request, in that I make this connection via web it works normal, however in this method it is presenting the 500 error in the connection.

public class CadastrarMarca extends AppCompatActivity {

HttpURLConnection urlConnection = null;
EditText editTextMarcaCadastro;
Button buttonSalvarMarca;
Marca marca = new Marca();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_cadastrar_marca);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    editTextMarcaCadastro = (EditText) findViewById(R.id.editTextMarcaCadastro);
    buttonSalvarMarca = (Button) findViewById(R.id.btnSalvarMarca);
    buttonSalvarMarca.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            marca.setNome(editTextMarcaCadastro.getText().toString());
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        URL url = new URL("http://192.168.73.110:8080/MrBar/rest/marca");
                        urlConnection = (HttpURLConnection) url.openConnection();
                        urlConnection.setRequestProperty("Content-Type", "application/json");
                        urlConnection.setRequestMethod("POST");
                        urlConnection.setRequestProperty("Accept", "application/json");
                        urlConnection.setRequestProperty("charset", "utf-8");
                        urlConnection.setUseCaches(false);
                        urlConnection.setDoOutput(true);
                        urlConnection.setInstanceFollowRedirects(false);
                        urlConnection.connect();
                        System.out.println(urlConnection.getResponseCode() + " Mensagem de sucesso ou falaha na conexao");
                        OutputStream outputStream = urlConnection.getOutputStream();
                        OutputStreamWriter streamWriter = new OutputStreamWriter(outputStream, "UTF-8");


                        Gson gson = new Gson();
                        String jsonMarca = gson.toJson(marca);
                        streamWriter.write(jsonMarca);
                        streamWriter.flush();
                        streamWriter.close();
                        outputStream.close();
                        urlConnection.disconnect();*/
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }).start();
        }
    });
}

}

Thank you in advance.

Follow the error below

  

SEVERE: Servlet.service () for servlet [br.com.mrbar.util.MrBarResourcesConfig] in context with path [/ MrBar] threw exception [java.lang.IllegalArgumentException: attempt to create saveOrUpdate event with null entity] with root cause   java.lang.IllegalArgumentException: attempt to create saveOrUpdate event with null entity