Getting the NoClassDefFoundError error!

package abdulrahman.com.stormy;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

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();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        String apiKey = "***";
        double latitude = 37;
        double longitude = -122;
        String forecastUrl = "https://api.forecast.io/forecast/"+apiKey+"/"+latitude+","+longitude;

        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 {
                    Log.v(TAG, response.body().string());
                    if (response.isSuccessful()) {
                    }
                    else {
                        alertUserAboutError();
                    }
                } catch (IOException e) {
                    Log.e(TAG, "Exception caught: ", e);
                }
            }
        });

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

    private void alertUserAboutError() {

        AlertDialogFragment dialog = new AlertDialogFragment();
        dialog.show(getFragmentManager(), "error");


    }
}

//and this is the class

public class AlertDialogFragment extends DialogFragment {

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        Context context = getActivity();
        AlertDialog.Builder builder = new AlertDialog.Builder(context)
                .setTitle("Oops!")
                .setMessage("Error!")
                .setPositiveButton("Sorry!",null);
        AlertDialog dialog = builder.create();
        return dialog;

    }
}