AlertDialog Error message keeps on appearing

package ronitmankad.com.stormy;

import android.content.Context;
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;

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

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

        String apiKey = "8565fa7afca9865c4475ddd7a84e9c81";
        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(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()) {
                            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_unavailable_error, 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);
        return new 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");
    }
}