Error NullPointerException .

public static final String TAG = MainActivity.class.getSimpleName();

private CurrentWeather mCurrentWeather;

@InjectView(R.id.timeLabel) TextView mTimeLabel;
@InjectView(R.id.temperatureLabel) TextView mTemperatureLabel;
@InjectView(R.id.humidityValue) TextView mHumidityValue;
@InjectView(R.id.precipValue) TextView mPrecipValue;
@InjectView(R.id.summaryLabel) TextView mSummaryLabel;
@InjectView(R.id.iconImageView) ImageView mIconImageView;
@InjectView(R.id.refreshImageView) ImageView mRefreshImageView;
@InjectView(R.id.progressBar) ProgressBar mProgressBar;

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

    mProgressBar.setVisibility(View.INVISIBLE);

    final double latitude = 37.8267;
    final double longitude = -122.423;

    mRefreshImageView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            getForecast(latitude, longitude);
        }
    });

    getForecast(latitude, longitude);

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

private void getForecast(double latitude, double longitude) {
    String apiKey = "27974c4bc33201748eaf542a6769c3b7";
    String forecastUrl = "https://api.forecast.io/forecast/" + apiKey +
            "/" + latitude + "," + longitude;

    if (isNetworkAvailable()) {
        toggleRefresh();

        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) {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        toggleRefresh();
                    }
                });
                alertUserAboutError();
            }

            @Override
            public void onResponse(Response response) throws IOException {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        toggleRefresh();
                    }
                });

                try {
                    String jsonData = response.body().string();
                    Log.v(TAG, jsonData);
                    if (response.isSuccessful()) {
                        mCurrentWeather = getCurrentDetails(jsonData);
                        runOnUiThread(new Runnable() {
                            @Override
                            public void run() {
                                updateDisplay();
                            }
                        });
                    } 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_is_unavailable_, Toast.LENGTH_LONG).show();
    }
}

private void toggleRefresh() {
    if (mProgressBar.getVisibility() == View.INVISIBLE) {
        mProgressBar.setVisibility(View.VISIBLE);
        mRefreshImageView.setVisibility(View.INVISIBLE);
    }
    else {
        mProgressBar.setVisibility(View.INVISIBLE);
        mRefreshImageView.setVisibility(View.VISIBLE);
    }
}

private void updateDisplay() {
    mTemperatureLabel.setText(mCurrentWeather.getTemperature() + "");
    mTimeLabel.setText("At " + mCurrentWeather.getFormattedDate() + " it will be");
    mHumidityValue.setText(mCurrentWeather.getHumidity() + "");
    mPrecipValue.setText(mCurrentWeather.getPrecipChance() + "%");
    mSummaryLabel.setText(mCurrentWeather.getSummary());

    Drawable drawable = getResources().getDrawable(mCurrentWeather.getIconId());
    mIconImageView.setImageDrawable(drawable);
}

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.setTime(currently.getLong("time"));
    currentWeather.setIcon(currently.getString("icon"));
    currentWeather.setPrecipChance(currently.getDouble("precipProbability"));
    currentWeather.setSummary(currently.getString("summary"));
    currentWeather.setTemperature(currently.getDouble("temperature"));
    currentWeather.setTimeZone(timezone);

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

    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;
    }

    return isAvailable;
}

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