my app stopped working out of nothing is giving an error "unfortunately app has stopped"

M \ app is working fine, so I turned on the pc and went to run it again and is giving this error "unfortunately app has stopped" I do not know what happened I'm going through the logcat here if someone knows what happened .

05-30 01:59:03.429 10960-10960/com.rafaeljacinto.newtest3 E/Trace: error opening trace file: No such file or directory (2)
05-30 01:59:04.049 10960-10960/com.rafaeljacinto.newtest3 E/dalvikvm: Could not find class 'android.graphics.drawable.RippleDrawable', referenced from method android.support.v7.widget.AppCompatImageHelper.hasOverlappingRendering
05-30 01:59:04.839 10960-11011/com.rafaeljacinto.newtest3 E/MainActivity: Response from url: {"dados":[{"nomeAlu":"Antonio Marins","emailAlu":"[email protected]","foneAlu":"53-99887766","codDisc":"15","nomeDisc":"Redes II","codProf":"1","nota1":"7","nota2":"8","media":"7.5","exame":"0","notafinal":"0","nomeProf":"Henry Carvalho","emailProf":"[email protected]"},{"nomeAlu":"Antonio Marins","emailAlu":"[email protected]","foneAlu":"53-99887766","codDisc":"19","nomeDisc":"Projeto Web II","codProf":"3","nota1":"7","nota2":"7","media":"7","exame":"0","notafinal":"0","nomeProf":"Everton Heckler","emailProf":"[email protected]"},{"nomeAlu":"Antonio Marins","emailAlu":"[email protected]","foneAlu":"53-99887766","codDisc":"26","nomeDisc":"Inteligencia Artificial","codProf":"4","nota1":"6","nota2":"8","media":"7","exame":"0","notafinal":"0","nomeProf":"Fabio Jose Paz","emailProf":"[email protected]"},{"nomeAlu":"Antonio Marins","emailAlu":"[email protected]","foneAlu":"53-99887766","codDisc":"28","nomeDisc":"Computador e Sociedade","codProf":"7","nota1":"10","nota2":"7","media":"8.5","exame":"0","notafinal":"0","nomeProf":"Paula Silveira","emailProf":"[email protected]"},{"nomeAlu":"Antonio Marins","emailAlu":"[email protected]","foneAlu":"53-99887766","codDisc":"28","nomeDisc":"Computador e Sociedade","codProf":"7","nota1":"6.5","nota2":"6","media":"6.25","exame":"5.75","notafinal":"6","nomeProf":"Paula Silveira","emailProf":"[email protected]"}]}
05-30 01:59:04.959 10960-10960/com.rafaeljacinto.newtest3 E/AndroidRuntime: FATAL EXCEPTION: main
    java.lang.NullPointerException
    at com.rafaeljacinto.newtest3.MainActivity$GetContacts.onPostExecute(MainActivity.java:155)
    at com.rafaeljacinto.newtest3.MainActivity$GetContacts.onPostExecute(MainActivity.java:55)
    at android.os.AsyncTask.finish(AsyncTask.java:631)
    at android.os.AsyncTask.access$600(AsyncTask.java:177)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4960)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:798)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:565)
    at dalvik.system.NativeStart.main(Native Method)

public class MainActivity extends AppCompatActivity {

private String TAG = MainActivity.class.getSimpleName();

private ProgressDialog progressDialog;
private ListView listView;
private TextView txtAluno;

String Aluno;
int code = 11111;

// JSON data url
//private static String Jsonurl = "http://192.168.0.12/ProjetoNewWebService/Dados.php?codAlu="+code;
ArrayList<HashMap<String, String>> contactJsonList;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    txtAluno = (TextView)findViewById(R.id.txtAluno);
    contactJsonList = new ArrayList<>();
    new GetContacts().execute();


}


private class GetContacts extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Showing progress dialog
        progressDialog = new ProgressDialog(MainActivity.this);
        progressDialog.setMessage("Por Favor Aguarde...");
        progressDialog.setCancelable(false);
        progressDialog.show();
    }

    @Override
    protected Void doInBackground(Void... arg0) {
        HttpHandler httpHandler = new HttpHandler();

        // request to json data url and getting response
        String Jsonurl = "http://192.168.0.12/ProjetoNewWebService/Dados.php?codAlu="+code;
        String jsonString = httpHandler.makeServiceCall(Jsonurl);
        Log.e(TAG, "Response from url: " + jsonString);
        if (jsonString != null) {
            try {
                JSONObject jsonObject = new JSONObject(jsonString);
                // Getting JSON Array node
                JSONArray contacts = jsonObject.getJSONArray("dados");

                for (int i = 0; i < contacts.length(); i++) {
                    JSONObject c = contacts.getJSONObject(i);
                    Aluno = c.getString("nomeAlu");
                    String nomeDisc = c.getString("nomeDisc");
                    String nomeProf = c.getString("nomeProf");
                    String nota1 = c.getString("nota1");
                    String nota2 = c.getString("nota2");
                    String media = c.getString("media");
                    String exame = c.getString("exame");
                    String notaFinal = c.getString("notafinal");


                    // tmp hash map for single contact
                    HashMap<String, String> contact = new HashMap<>();

                    // adding each child node to HashMap key => value
                    contact.put("nomeAlu", Aluno);
                    contact.put("name", nomeDisc);
                    contact.put("professor", nomeProf);
                    contact.put("nota1", nota1);
                    contact.put("nota2", nota2);
                    contact.put("media", media);
                    contact.put("exame", exame);
                    contact.put("notafinal", notaFinal);

                    // adding contact to contact list
                    contactJsonList.add(contact);

                }
            } catch (final JSONException e) {
                Log.e(TAG, "Json parsing error: " + e.getMessage());
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(getApplicationContext(),
                                "Json parsing error: " + e.getMessage(),
                                Toast.LENGTH_LONG)
                                .show();
                    }
                });

            }

        } else {
            Log.e(TAG, "Could not get json from server.");
            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    Toast.makeText(getApplicationContext(),
                            "Could not get json from server.",
                            Toast.LENGTH_LONG)
                            .show();
                }
            });

        }

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        super.onPostExecute(result);
        // Dismiss the progress dialog
        if (progressDialog.isShowing())
            progressDialog.dismiss();
        /**
         * Updating parsed JSON data into ListView
         * */
        ListAdapter adapter = new SimpleAdapter(
                MainActivity.this, contactJsonList,
                R.layout.list_item, new String[]{"name", "professor","nota1","nota2","media","exame","notafinal"}, new int[]{R.id.name,
                R.id.professor, R.id.nota1, R.id.nota2, R.id.media, R.id.exame, R.id.notafinal });

        listView.setAdapter(adapter);
        txtAluno.setText(Aluno);
    }

}

}