title: NullPointerException: Attempt to invoke virtual method AlertDialog.setTitle(java.lang.CharSequence) on a null object reference
body: <p><strong>Here is the error:</strong></p>
  
  <blockquote>
    <p>04-01 10:37:47.077    2310-2326/zonup.asyc D/Create Response﹕
    {"success":"false","msg":"Please enter email!!!"} 04-01 10:37:47.084<br>
    2310-2310/zonup.asyc D/AndroidRuntime﹕ Shutting down VM 04-01
    10:37:47.084    2310-2310/zonup.asyc E/AndroidRuntime﹕ FATAL
    EXCEPTION: main
        Process: zonup.asyc, PID: 2310
        java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.AlertDialog.setTitle(java.lang.CharSequence)' on a
    null object reference
                at zonup.asyc.MainActivity$SignmeUp.onPostExecute(MainActivity.java:122)
                at zonup.asyc.MainActivity$SignmeUp.onPostExecute(MainActivity.java:70)
                at android.os.AsyncTask.finish(AsyncTask.java:632)
                at android.os.AsyncTask.access$600(AsyncTask.java:177)
                at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
                at android.os.Handler.dispatchMessage(Handler.java:102)
                at android.os.Looper.loop(Looper.java:135)
                at android.app.ActivityThread.main(ActivityThread.java:5221)
                at java.lang.reflect.Method.invoke(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:372)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)</p>
  </blockquote>
  
  <pre><code>public class MainActivity extends ActionBarActivity {
  
  protected String stmail;
  protected EditText editText;
  protected Button button;
  AlertDialog alertDialog;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      editText=(EditText) findViewById(R.id.editText);
      stmail=editText.getText().toString();
      button=(Button) findViewById(R.id.button);
      button.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View v) {
              new SignmeUp().execute();
          }
      });
  }
  
  
  @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, menu);
      return true;
  }
  
  @Override
  public boolean onOptionsItemSelected(MenuItem item) {
      // Handle action bar item clicks here. The action bar will
      // automatically handle clicks on the Home/Up button, so long
      // as you specify a parent activity in AndroidManifest.xml.
      int id = item.getItemId();
  
      //noinspection SimplifiableIfStatement
      if (id == R.id.action_settings) {
          return true;
      }
  
      return super.onOptionsItemSelected(item);
  }
  class SignmeUp extends AsyncTask&lt;String, String, String&gt; {
  
      private ProgressDialog pDialog;
      JSONParser jsonParser = new JSONParser();
  
      @Override
      protected void onPreExecute() {
          super.onPreExecute();
          runOnUiThread(new Runnable() {
              @Override
              public void run() {
                  pDialog = new ProgressDialog(MainActivity.this);
                  pDialog.setMessage("Thanks for your Sign up...");
                  pDialog.setIndeterminate(false);
                  pDialog.setCancelable(true);
              }
          });
      }
  
      @Override
      protected String doInBackground(String... arg0) {
          // TODO Auto-generated method stub
  
          // Building Parameters
          List&lt;NameValuePair&gt; params = new ArrayList&lt;NameValuePair&gt;();
          params.add(new BasicNameValuePair("email", stmail));
  
  
  
          // getting JSON Object
          // Note that create product url accepts POST method
          JSONObject json = jsonParser.makeHttpRequest("http://api.php","GET", params);
  
          // check log cat fro response
          Log.d("Create Response", json.toString());
  
          // check for success tag
          try {
              String success = json.getString("success");
  
              if (stmail != null &amp; success == "true") {
                  Toast.makeText(MainActivity.this, "Youre Email Posted..", Toast.LENGTH_SHORT).show();
              } else {
  
              }
          } catch (JSONException e) {
              e.printStackTrace();
          }
  
          return null;
      }
  
      @SuppressWarnings("deprecation")
      protected void onPostExecute(String file_url) {
          // dismiss the dialog once done
  
          editText.setText("");
          alertDialog.setTitle("Info");
  
          alertDialog.setMessage("You have been subscribed o the ");
          alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
              public void onClick(DialogInterface dialog, int which) {
                  // here you can add functions
  
              }
          });
  
          alertDialog.show();
  
      }
  
  }
  }
  </code></pre>