Android Finish() an Activity in Asynctask
<p>I'm trying to close an app if there is no internet connection using connection manager. However after i implemented those codes. I cant seem to get it run, it just crashes with an exception. Below are my codes and my logcat .</p>

<pre><code> public class BuyFragment extends SherlockListFragment implements
        ActionBar.TabListener {

    static final String URL = "https://myxml.xml";
    static final String KEY_SONG = "song";
    static final String KEY_ID = "id";
    static final String KEY_TITLE = "title";
    static final String KEY_CAT_ARTIST = "artistcat";
    static final String KEY_DURATION = "duration";
    static final String KEY_THUMB_URL = "thumb_url";
    static final String KEY_BIG_URL = "big_url";
    static final String KEY_CAT_URL = "cat_url";
    static final String KEY_DESC = "cat_desc";
    ArrayList&lt;HashMap&lt;String, String&gt;&gt; menuItems;
    ListAdapter adapter;
    Context appContext;
    Activity mAct;
    private Fragment mFragment;

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {

        if (!isNetworkAvailable()) {
            Toast.makeText(getActivity(),
                    "Internet Connection Required", Toast.LENGTH_SHORT).show();
            //this.finish();
            mAct.finish();
        } else { 
        new loadListView().execute();

        }
        super.onActivityCreated(savedInstanceState);
    }


     private boolean isNetworkAvailable() {
         ConnectivityManager connectivityManager = (ConnectivityManager) getActivity().getSystemService(Context.CONNECTIVITY_SERVICE);
         NetworkInfo activeNetworkInfo = connectivityManager
                 .getActiveNetworkInfo();
         return activeNetworkInfo != null;
     }

    public class loadListView extends AsyncTask&lt;Integer, String, String&gt; {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected String doInBackground(Integer... args) {
            // updating UI from Background Thread
            menuItems = new ArrayList&lt;HashMap&lt;String, String&gt;&gt;();

            XMLParser parser = new XMLParser();
            String xml = parser.getXmlFromUrl(URL); // getting XML
            Document doc = parser.getDomElement(xml); // getting DOM element

            NodeList nl = doc.getElementsByTagName(KEY_SONG);
            // looping through all item nodes &lt;item&gt;
            for (int i = 0; i &lt; nl.getLength(); i++) {
                // creating new HashMap
                HashMap&lt;String, String&gt; map = new HashMap&lt;String, String&gt;();
                Element e = (Element) nl.item(i);
                // adding each child node to HashMap key =&gt; value
                map.put(KEY_ID, parser.getValue(e, KEY_ID));
                map.put(KEY_TITLE, parser.getValue(e, KEY_TITLE));
                map.put(KEY_CAT_ARTIST, parser.getValue(e, KEY_CAT_ARTIST));
                map.put(KEY_DURATION, parser.getValue(e, KEY_DURATION));
                map.put(KEY_THUMB_URL, parser.getValue(e, KEY_THUMB_URL));
                map.put(KEY_BIG_URL, parser.getValue(e, KEY_BIG_URL));
                map.put(KEY_CAT_URL, parser.getValue(e, KEY_CAT_URL));
                map.put(KEY_DESC, parser.getValue(e, KEY_DESC));
                // adding HashList to ArrayList
                menuItems.add(map);
            }
            return null;
        }

        @Override
        protected void onPostExecute(String args) {

            if (getActivity() != null) {
                adapter = new MainPageLazyAdapter(getActivity(), menuItems);
                setListAdapter(adapter);

                ListView lv = getListView();

                lv.setOnItemClickListener(new OnItemClickListener() {

                    @Override
                    public void onItemClick(AdapterView&lt;?&gt; parent, View view,
                            int position, long id) {
                        String thumburl;
                        String bigurl;
                        String caturl;
                        String title;
                        String desc;
                        String artist;
                        Intent in;

                        title = ((TextView) view.findViewById(R.id.title))
                                .getText().toString();
                        artist = ((TextView) view.findViewById(R.id.artist))
                                .getText().toString();
                        desc = ((TextView) view.findViewById(R.id.desc))
                                .getText().toString();
                        thumburl = ((TextView) view
                                .findViewById(R.id.thumb_image)).getText()
                                .toString();
                        caturl = ((TextView) view.findViewById(R.id.cat))
                                .getText().toString();
                        bigurl = ((TextView) view.findViewById(R.id.big_image))
                                .getText().toString();

                        in = new Intent(getActivity(),
                                SingleImageViewActivity.class);

                        in.putExtra(KEY_TITLE, title);
                        in.putExtra(KEY_DESC, desc);
                        in.putExtra(KEY_CAT_URL, caturl);
                        in.putExtra(KEY_THUMB_URL, thumburl);
                        in.putExtra(KEY_BIG_URL, bigurl);
                        in.putExtra(KEY_CAT_ARTIST, artist);
                        startActivity(in);

                    }
                });
            }

        }
    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        mFragment = new BuyFragment();
        ft.add(android.R.id.content, mFragment);
        ft.attach(mFragment);
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        ft.remove(mFragment);
    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {

    }
}
</code></pre>

<p>LOGCAT :</p>

<pre><code>11-02 14:56:48.390: E/AndroidRuntime(13446): FATAL EXCEPTION: main
11-02 14:56:48.390: E/AndroidRuntime(13446): java.lang.NullPointerException
11-02 14:56:48.390: E/AndroidRuntime(13446):    at in.wptrafficanalyzer.actionbarsherlocknavtab.BuyFragment.onActivityCreated(BuyFragment.java:57)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:891)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1080)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:622)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1416)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:420)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.os.Handler.handleCallback(Handler.java:587)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.os.Handler.dispatchMessage(Handler.java:92)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.os.Looper.loop(Looper.java:130)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at android.app.ActivityThread.main(ActivityThread.java:3691)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at java.lang.reflect.Method.invokeNative(Native Method)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at java.lang.reflect.Method.invoke(Method.java:507)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:907)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:665)
11-02 14:56:48.390: E/AndroidRuntime(13446):    at dalvik.system.NativeStart.main(Native Method)
</code></pre>
