Android Weather app Course - Fatal Error

package rvb.weather.stormy;

import android.content.Context;
import android.databinding.DataBindingUtil;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;

import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.internal.tls.OkHostnameVerifier;
import rvb.weather.stormy.databinding.ActivityMainBinding;

public class MainActivity extends AppCompatActivity {

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final ActivityMainBinding binding = DataBindingUtil.setContentView(MainActivity.this.R.layout.activity_main);

        String apiKey = "57eaf3aa961968bf65b0619680588073";
        double latitude = 37.8267;
        double longitude = -122.4233;

        String forecastURL = "https://api.darksky.net/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(Call call, IOException e) {

                }

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

                            CurrentWeather displayWeather = new CurrentWeather(
                                    currentWeather.getLocationLabel(),
                                    currentWeather.getIcon(),
                                    currentWeather.getTime(),
                                    currentWeather.getTemperature(),
                                    currentWeather.getHumidity(),
                                    currentWeather.getPrecipChance(),
                                    currentWeather.getSummary(),
                                    currentWeather.getTimeZone()
                            );

                            binding.setWeather(displayWeather);

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

                }
            });
        }
        else {
            Toast.makeText(this, R.string.network_unavailable_message,
                    Toast.LENGTH_LONG).show();
        }
        Log.d(TAG, "Main UI code is running, hooray!");
    }

    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();

        // Parse weather data from currently object
        currentWeather.setHumidity(currently.getDouble("humidity"));
        currentWeather.setTime(currently.getLong("time"));
        currentWeather.setIcon(currently.getString("icon"));
        currentWeather.setLocationLabel("Alcatraz Island, CA");
        currentWeather.setPrecipChance(currently.getDouble("precipProbability"));
        currentWeather.setSummary(currently.getString("summary"));
        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;
        }

        return isAvailable;
    }

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