title: NullPointer after execution onPostExecute method
body: <p>I have a problem with retrieving JSON. My JSON looks like this:</p>
  
  <pre><code>[{"Kli_nazwa":"miechy ","Kli_adres":"miechowity ","Kli_nip":"12"},{"Kli_nazwa":"mi? ","Kli_adres":"szp 39","Kli_nip":"19"}]
  </code></pre>
  
  <p>The app is throwing a NullPointerException in onPostExecute, even if thr JSON was retrieved correctly. What it's going on? I am totally stuck.</p>
  
  <pre><code>private class PHPConnection_Klient extends AsyncTask&lt;String, String, String&gt; {
  
      @Override
      protected String doInBackground(String... params) {
  
          wynik_klienci = getResponseKli();
  
          return wynik_klienci;
      }
  
      @Override
      protected void onPostExecute(String wynik_klienci) {
          // TODO Auto-generated method stub
          super.onPostExecute(wynik_klienci);
          System.out.println(wynik_klienci);
  
          Gson g1 = new Gson();
  
          List&lt;Klient&gt; klient = Arrays.asList(g1.fromJson(wynik_klienci,
                  Klient[].class));
  
          try {
              klientDao.delete(klientDao.queryForAll());
  
              if (klient != null) {
                  for (Klient k : klient) {
                      klientDao.create(k);
                  }
              }
  
          } catch (final SQLException e) {
              runOnUiThread(new Runnable() {
  
                  @Override
                  public void run() {
                      e.printStackTrace();
                  }
              });
          }
      }
  
      @Override
      protected void onPreExecute() {
          // TODO Auto-generated method stub
          super.onPreExecute();
      }
  }
  
  private String getResponseKli() {
      HttpClient client = new DefaultHttpClient();
  
      HttpGet request1 = new HttpGet(
              "myURL");
  
      try {
          HttpResponse response1 = client.execute(request1);
  
          String w1 = EntityUtils.toString(response1.getEntity());
  
          return w1;
      } catch (ClientProtocolException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
      } catch (IOException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
      }
  
      return "";
  }
  </code></pre>
  
  <p>Errors:</p>
  
  <pre><code>03-19 10:29:06.395: W/dalvikvm(5851): threadid=1: thread exiting with uncaught  exception (group=0x40be01f8)
  03-19 10:29:06.403: E/AndroidRuntime(5851): FATAL EXCEPTION: main
  03-19 10:29:06.403: E/AndroidRuntime(5851): java.lang.NullPointerException
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at  com.android.internal.os.LoggingPrintStream.println(LoggingPrintStream.java:298)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at com.example.inzynierka.MainActivity$PHPConnection_Klient.onPostExecute(MainActivity.java:627)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at com.example.inzynierka.MainActivity$PHPConnection_Klient.onPostExecute(MainActivity.java:1)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at android.os.AsyncTask.finish(AsyncTask.java:602)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at android.os.AsyncTask.access$600(AsyncTask.java:156)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:615)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at android.os.Handler.dispatchMessage(Handler.java:99)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at android.os.Looper.loop(Looper.java:137)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at android.app.ActivityThread.main(ActivityThread.java:4514)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at java.lang.reflect.Method.invokeNative(Native Method)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at java.lang.reflect.Method.invoke(Method.java:511)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
  03-19 10:29:06.403: E/AndroidRuntime(5851):     at dalvik.system.NativeStart.main(Native Method)
  </code></pre>
  
  <p>And this is how i'm executing two PHPConnections:</p>
  
  <pre><code>PHPConnection_Klient php_kli = new PHPConnection_Klient();
          php_kli.execute();
          PHPConnection_Kategoria php_kt = new PHPConnection_Kategoria();
  
          php_kt.execute();
  </code></pre>