title: ProgressDialogue throwing NullPointerException
body: <p>i am using ProgressDialogue with AsyncTask but NullPointerException is occurring.</p>
  
  <p><strong>Code</strong></p>
  
  <pre><code>public class MainActivity extends Activity {
  
      //private MyProgressDialog dialog;
      @Override
      public void onCreate(Bundle icicle) {
  
          super.onCreate(icicle);
  
          setContentView(R.layout.activity_main);
  
          new ProgressTask(MainActivity.this).execute();
     }
  
  
    /**
   * this class performs all the work, shows dialog before the work and dismiss it after
   */
  public class ProgressTask extends AsyncTask&lt;String, Void, Boolean&gt; {
  
      private Activity context;
  
      public ProgressTask(Activity mainActivity) {
          this.activity = mainActivity;
          dialog = new ProgressDialog(context);
      }
  
      /** progress dialog to show user that the backup is processing. */
      private ProgressDialog dialog;
      /** application context. */
      private Activity activity;
  
      protected void onPreExecute() {
          this.dialog.setMessage("Progress start");
          this.dialog.show();
      }
  
          @Override
      protected void onPostExecute(final Boolean success) {
          if (dialog.isShowing()) {
              dialog.dismiss();
          }
   }
  
      protected Boolean doInBackground(final String... args) {
         try{    
  
            return true;
         } catch (Exception e){
            Log.e("tag", "error", e);
            return false;
         }
      }
  }
  }
  </code></pre>