I get an error when running the app.

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

    String apiKey = "06df8f38f75dbddb7b62d8dc9d734599";
    double latitude = 37.8267;
    double longitude = 122.423;
    String forecastUrl = "https://api.forecast.io/forecast/" + apiKey +
            "/" + latitude + "," + longitude;

    if (isNetworkAvailable()) {
        OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder()
                .url(forecastUrl)
                .build();

        Call call = client.newCall(request);
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Request request, IOException e) {

            }

            @Override
            public void onResponse(Response response) throws IOException {
                try {
                    String jsonData = response.body().string();
                    Log.v(TAG,jsonData );
                    if (response.isSuccessful()) {

                        mCurrentWeather = getCurrentDetails(jsonData);

                    } else {
                        alertUserAboutError();
                    }
                }
                catch (IOException e) {
                    Log.e(TAG, "Exception caught", e);
                }
                catch ( JSONException e) {
                    Log.e(TAG, "Exception caught", e);

                }
            }
        });
    }
    else {
        Toast.makeText(this, R.string.network_unaviable_message,
                Toast.LENGTH_LONG ).show();
    }

    Log.d(TAG, "Main UI code is running!");

}

private CurrentWeather getCurrentDetails(String jsonData) throws JSONException{
    JSONObject forecast = new JSONObject(jsonData);
    String timezone = forecast.getString("timezone");
    Log.i(TAG, "From JSON:" + timezone);

    JSONObject currently = forecast.getJSONObject("currently");

    CurrentWeather currentWeather = new CurrentWeather();
    currentWeather.setHumidity(currently.getDouble("Humidity"));
    currentWeather.getTime(currently.getLong("time"));
    currentWeather.setIcon(currently.getString("icon"));
    currentWeather.setPrecipChance(currently.getDouble("precipProbability"));
    currentWeather.setTemperature(currently.getDouble("temperature"));
    currentWeather.setTimeZone(timezone);

    Log.d(TAG, currentWeather.getFormattedTime());

    return currentWeather;

private boolean isNetworkAvailable() {
    ConnectivityManager manager = (ConnectivityManager)
            getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = manager.getActiveNetworkInfo();
    boolean isAvailable = false;
    if (networkInfo != null && networkInfo.isConnected()) {
       isAvailable = true;


    }

private void alertUserAboutError() {
    AlertDialogFragment dialog = new AlertDialogFragment();
    dialog.show(getFragmentManager(), "error_dialog");

}