Android Studio - Alert Dialog does not appear

protected String[] mBlogPostTitles;
public static final int NUMBER_OF_POSTS = 20;
public static final String TAG = MainListActivity.class.getSimpleName();
protected JSONObject mBlogData;
protected ProgressBar mProgressBar;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main_list);

    mProgressBar = (ProgressBar) findViewById(R.id.progressBar);

    if(isNetworkAvailable()){
        mProgressBar.setVisibility(View.VISIBLE);
        GetBlogPostsTask getBlogPostsTask = new GetBlogPostsTask();
        getBlogPostsTask.execute();
    }
    else{
        Toast.makeText(this,"Network is unavailable",Toast.LENGTH_LONG);
    }

    //Toast.makeText(this, getString(R.string.no_items);, Toast.LENGTH_LONG).show();

}

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;
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main_list, menu);
    return true;
}

private void handleBlogResponse() {
    mProgressBar.setVisibility(View.INVISIBLE);

    if(mBlogData == null){
        updateDisplayForError();
    }
    else{
        try {
            JSONArray jsonPosts = mBlogData.getJSONArray("posts");
            mBlogPostTitles = new String[jsonPosts.length()];
            for(int i = 0; i < jsonPosts.length();i++){
                JSONObject post = jsonPosts.getJSONObject(i);
                String title = post.getString("title");
                title = Html.fromHtml(title).toString();
                mBlogPostTitles[i] = title;
            }

            ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,mBlogPostTitles);
            setListAdapter(adapter);
        } catch (JSONException e) {
            Log.e(TAG,"Exception Caught!",e);
        }
    }
}

private void updateDisplayForError() {
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setTitle(getString(R.string.error_title));
    builder.setMessage(getString(R.string.error_message));
    builder.setPositiveButton(android.R.string.ok,null);
    AlertDialog dialog = builder.create();
    dialog.show();

    TextView emptyTextView = (TextView) getListView().getEmptyView();
    emptyTextView.setText(getString(R.string.no_items));
}

private class GetBlogPostsTask extends AsyncTask<Object, Void, JSONObject> {

    @Override
    protected JSONObject doInBackground(Object[] params) {
        int responseCode = -1;
        JSONObject jsonResponse = null;
        try{
            URL blogFeedUrl = new URL("http://blog.teamtreehouse.com/api/get_recent_summary/?count="+NUMBER_OF_POSTS);
            HttpURLConnection connection = (HttpURLConnection) blogFeedUrl.openConnection();
            connection.connect();

            responseCode = connection.getResponseCode();
            if(responseCode == HttpURLConnection.HTTP_OK){
                InputStream inputStream = connection.getInputStream();
                Reader reader = new InputStreamReader(inputStream);
                int nextCharacter; // read() returns an int, we cast it to char later
                String responseData = "";
                while(true){ // Infinite loop, can only be stopped by a "break" statement
                    nextCharacter = reader.read(); // read() without parameters returns one character
                    if(nextCharacter == -1) // A return value of -1 means that we reached the end
                        break;
                    responseData += (char) nextCharacter; // The += operator appends the character to the end of the string
                }
                jsonResponse = new JSONObject(responseData);

            }
            else {
                Log.i(TAG, "Unsuccessful HTTP Response Code: " + responseCode);
            }
        }
        catch(MalformedURLException e){
            Log.e(TAG, "Exception caught: ", e);
        } catch (IOException e) {
            Log.e(TAG, "Exception caught: ", e);
        } catch (Exception e){
            Log.e(TAG, "Exception caught: ", e);
        }

        return jsonResponse;
    }


    @Override
    protected void onPostExecute(JSONObject result) {
        mBlogData = result;
        handleBlogResponse();
    }
}