Android HeadFirst Error

package id.mediatutorial.nasadailyimages;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ImageView;
import android.widget.TextView;

import org.xml.sax.Attributes;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.DefaultHandler;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

public class MainActivity extends AppCompatActivity {
    IotdHandler handler = new IotdHandler();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        /****** IotdHandler *****/
        handler.processFeed();

    }

    public void ResetDisplay() {

        String title = handler.getTitle();
        String date = handler.getDate();
        String description = handler.getDescription().toString();

        resetDisplay(title, date, handler.getImage(), description);
    }

    private void resetDisplay(String title, String date, Bitmap image, String description) {
        try {
            TextView titleView = (TextView) findViewById(R.id.imageTitle);
            titleView.setText(title);
            TextView dateView = (TextView) findViewById(R.id.imageDate);
            dateView.setText(date);
            ImageView imageView = (ImageView) findViewById(R.id.imageDisplay);
            imageView.setImageBitmap(image);
            TextView descriptionView = (TextView) findViewById(R.id.imageDescription);
            descriptionView.setText(description);
        }catch(Exception e){
            TextView titleView = (TextView) findViewById(R.id.imageTitle);
            titleView.setText(e.toString());
        }

    }

    public class IotdHandler extends DefaultHandler {
        private String url = "http://www.nasa.gov/rss/image_of_the_day.rss";
        private boolean inUrl = false;
        private boolean inTitle = false;
        private boolean inDescription = false;
        private boolean inItem = false;
        private boolean inDate = false;
        private Bitmap image = null;
        private String title = null;
        private StringBuffer description = new StringBuffer();
        private String date = null;

        public XMLReader reader = null;

        private Bitmap getBitmap(String url) {
            try {
                HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
                connection.setDoInput(true);
                connection.connect();

                InputStream input = connection.getInputStream();
                Bitmap bitmap = BitmapFactory.decodeStream(input);

                input.close();

                return bitmap;

            } catch (IOException ioe) {
                TextView titleView = (TextView) findViewById(R.id.imageTitle);
                titleView.setText(ioe.toString());
                return null;
            }
        }

        public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
            if (localName.equals("enclosure")) {
                inUrl = true;
            } else {
                inUrl = false;
            }

            if (localName.startsWith("item")) {
                inItem = true;
            } else if (inItem) {
                if (localName.equals("title")) {
                    inTitle = true;
                } else {
                    inTitle = false;
                }

                if (localName.equals("description")) {
                    inDescription = true;
                } else {
                    inDescription = false;
                }

                if (localName.equals("pubDate")) {
                    inDate = true;
                } else {
                    inDate = false;
                }
            }
        }

        public void characters(char ch[], int start, int length) {
            String chars = new String(ch).substring(start, start + length);
            if (inTitle && title == null) {
                title = chars;
            }
            if (inDescription) {
                description.append(chars);
            }
            if (inDate && date == null) {
                date = chars;
            }
        }

        private class ProcessFeedTask extends AsyncTask<String, Void, InputStream> {
            @Override
            protected InputStream doInBackground(String... params) {
                String url = params[0];

                InputStream inputStream = null;

                try {
                    inputStream = new URL(url).openStream();

                    reader.parse(new InputSource(inputStream));

                } catch (Exception e) {
                    //TextView titleView = (TextView) findViewById(R.id.imageTitle);
                    //titleView.setText(e.toString());
                    //
                    //i comment out two line above becose it always display error:
                    // //method setText must be called from UI Thread
                }

                return inputStream;
            }

            @Override
            protected void onPostExecute(InputStream result) {
                super.onPostExecute(result);

                if (result != null) {
                    ResetDisplay();
                }
            }
        }

        public void processFeed()
        {
            try
            {
                SAXParserFactory factory = SAXParserFactory.newInstance();

                SAXParser parser = factory.newSAXParser();

                reader = parser.getXMLReader();
                reader.setContentHandler(this);

                new ProcessFeedTask().execute(url);

            }
            catch (Exception e)
            {
                TextView titleView = (TextView) findViewById(R.id.imageTitle);
                titleView.setText(e.toString());
            }
        }

        public Bitmap getImage() {
            return image;
        }

        public String getTitle() {
            return title;
        }

        public String getDescription() {
            return description.toString();
        }

        public String getDate() {
            return date;
        }
    }
}