сохранение веб-страниц в кэш webview в android

Я создаю приложение, которое будет загружать некоторые веб-страницы из интернета и сохранять их в кэше, когда пользователь нажимает на кнопку, и загружать их позже, когда интернет недоступен. когда я запускаю свой код, я получаю исключение нулевого указателя. Я добавил все необходимые разрешения в файл манифеста.

public class MainActivity extends Activity {



Context context;

ArrayList<String> fileNames = new ArrayList<String>();

ArrayList<String> urls = new ArrayList<String>();

ArrayList<String> paths = new ArrayList<String>();

ArrayList<File> files = new ArrayList<File>();

boolean internet;



@Override

protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.mainscreen);



    Button saveSiteOne = (Button)findViewById(R.id.button1);

    saveSiteOne.setOnClickListener(new View.OnClickListener() {

       public void onClick(View v) {



        urls.add("http://www.google.com"); 

        urls.add("http://www.facebook.com"); 



        for(int i=0;i<urls.size();i++){

            fileNames.add("file"+i+".html");

        }



        //Loop to get paths of files

        for(int i=0;i<urls.size();i++){

            paths.add(getApplicationContext().getFilesDir() + "/" + fileNames.get(i));

        }



        //Loop to create list of files getting from stored paths

        for(int i=0;i<urls.size();i++){

            files.add(new File(paths.get(i)));

        }



        internet = isNetworkAvailable();



        WebView webView = new WebView(context);

        WebSettings webSettings = webView.getSettings();

        webView.getSettings().setAppCacheMaxSize( 5 * 1024 * 1024 ); // 5MB

        webView.getSettings().setAppCachePath( context.getCacheDir().getAbsolutePath() );

        webView.getSettings().setAllowFileAccess( true );

        webView.getSettings().setAppCacheEnabled( true );

        webView.getSettings().setJavaScriptEnabled( true );

        webView.getSettings().setCacheMode( WebSettings.LOAD_DEFAULT ); // load online by default



        new downloadContentOfPage().execute();





       }

   });

}





class downloadContentOfPage extends AsyncTask<String, Void, String> {



    FileOutputStream fos;

    String result = ""; 

    ProgressDialog progressDialog;

    int position;



    @Override

    protected void onPreExecute() {

        super.onPreExecute();



        progressDialog = new ProgressDialog(MainActivity.this);

        progressDialog.setMessage("Loading...");

        progressDialog.setCancelable(false);

        progressDialog.show();

        Log.e( "pre execute","yes"); 



        try {

            fos = context.openFileOutput(fileNames.get(position), Context.MODE_PRIVATE);

        }

        catch (IOException e) {

            e.printStackTrace();

        }

    }   



    @Override

    protected String doInBackground(String... params) {



        HttpClient httpClient = new DefaultHttpClient();

        HttpContext localContext = new BasicHttpContext();

        //give your url of html page that you want to download first time.

        HttpGet httpGet = new HttpGet(urls.get(position));

        HttpResponse response = null;

        try {

            response = httpClient.execute(httpGet, localContext);

        } catch (ClientProtocolException e) {

            e.printStackTrace();

        } catch (IOException e) {

            e.printStackTrace();

        }

        BufferedReader reader = null;

        try {

            reader = new BufferedReader(

                    new InputStreamReader(response.getEntity().getContent()

                            )

                    );

        } catch (IllegalStateException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        } catch (IOException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }



        String line = null;

        try {

            while ((line = reader.readLine()) != null){

                result += line + "n";

            }

        } catch (IOException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

        return result;

    }





    @Override

    protected void onPostExecute(String res) {



        try {

            fos.write(res.getBytes());

            fos.close();



        } catch (IOException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }



        Toast.makeText( context.getApplicationContext(),"saved", Toast.LENGTH_LONG).show();

        if(progressDialog.isShowing()) {

           progressDialog.dismiss();

        } 



    }

}

Logcat

03-05 17:07:08.269: E/AndroidRuntime(14974): FATAL EXCEPTION: main

03-05 17:07:08.269: E/AndroidRuntime(14974): java.lang.NullPointerException

03-05 17:07:08.269: E/AndroidRuntime(14974):    at android.view.ViewConfiguration.get(ViewConfiguration.java:318)

03-05 17:07:08.269: E/AndroidRuntime(14974):    at android.view.View.<init> (View.java:3254)

03-05 17:07:08.269: E/AndroidRuntime(14974):    at android.view.View.<init>(View.java:3299)

03-05 17:07:08.269: E/AndroidRuntime(14974):    at android.view.ViewGroup.<init>(ViewGroup.java:443)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at   android.widget.AbsoluteLayout.<init>(AbsoluteLayout.java:52)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.webkit.WebView.<init>(WebView.java:556)

  03-05 17:07:08.269: E/AndroidRuntime(14974):  at android.webkit.WebView.<init>(WebView.java:533)

  03-05 17:07:08.269: E/AndroidRuntime(14974):  at android.webkit.WebView.<init>(WebView.java:513)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.webkit.WebView.<init>(WebView.java:502)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.webkit.WebView.<init>(WebView.java:492)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at com.example.html.MainActivity$1.onClick(MainActivity.java:78)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.view.View.performClick(View.java:4222)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.view.View$PerformClick.run(View.java:17620)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.os.Handler.handleCallback(Handler.java:800)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.os.Handler.dispatchMessage(Handler.java:100)

 03-05 17:07:08.269: E/AndroidRuntime(14974):   at android.os.Looper.loop(Looper.java:194)

03-05 17:07:08.269: E/AndroidRuntime(14974):    at android.app.ActivityThread.main(ActivityThread.java:5370)

03-05 17:07:08.269: E/AndroidRuntime(14974):    at java.lang.reflect.Method.invokeNative(Native Method)

03-05 17:07:08.269: E/AndroidRuntime(14974):    at java.lang.reflect.Method.invoke(Method.java:525)