Unable to add window - token null is not valid; is your activity running?

The fact is that I have a ProgressDialog within a class AsyncTask and the case is that when it is executed, to make a connection, the first time I get the Progress correctly, but if I do a search again (this is what the Asynctask is responsible for) I get the following error:

  

FATAL EXCEPTION: main               Process: com.xxxx.cccc, PID: 25502                                                                    android.view.WindowManager $ BadTokenException: Unable to add window - token null is not valid; is your activity running?                   at android.view.ViewRootImpl.setView (ViewRootImpl.java:765)                   at android.view.WindowManagerGlobal.addView (WindowManagerGlobal.java:356)                   at android.view.WindowManagerImpl.addView (WindowManagerImpl.java:93)                   at android.app.Dialog.show (Dialog.java:330)                   at com.xxxx.cccc.conexion.CRC.onPreExecute (CRC.java:79)                   at android.os.AsyncTask.executeOnExecutor (AsyncTask.java:648)                   at android.os.AsyncTask.execute (AsyncTask.java:595)                   at com.xxxx.cccc.listas.LCNA $ 5.onClick (LCNA.java:127)                   at android.view.View.performClick (View.java:6294)                   at android.view.View $ PerformClick.run (View.java:24770)                   at android.os.Handler.handleCallback (Handler.java:790)                   at android.os.Handler.dispatchMessage (Handler.java:99)                   at android.os.Looper.loop (Looper.java:164)                   at android.app.ActivityThread.main (ActivityThread.java:6494)                   at java.lang.reflect.Method.invoke (Native Method)                   at com.android.internal.os.RuntimeInit $ MethodAndArgsCaller.run (RuntimeInit.java:438)                   at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:807)

This is the code:

public class xxxx extends AsyncTask<String, Void, String> {
...
private ProgressDialog progressDialog;
...
protected void onPreExecute() {
    super.onPreExecute();
    progressDialog = new ProgressDialog(contexto);
    progressDialog.setCanceledOnTouchOutside(false);
    progressDialog.setCancelable(true);
    progressDialog.setIndeterminate(false);
    progressDialog.setMessage("Conectando con el servidor. Espere por favor...");
    progressDialog.show();
}

@Override
protected String doInBackground(String... values) {
...
    if (conectarServidor()) {
        publishProgress();

        transferenciaInformacion(); //Recibe información del Servidor y lo trata

        this.cancel(true);

    } else {
        publishProgress();
    }


    return "true";
}

@Override
protected void onPostExecute(String s) {
    super.onPostExecute(s);
progressDialog.cancel();


}

 @Override
protected void onCancelled(String s) {
progressDialog.cancel();
    super.onCancelled(s);
}

    @Override
protected void onProgressUpdate(Void... values) {
    super.onProgressUpdate(values);
    /**
     * Condición de que no haya logrado conectarse con el servidor
     * En cuyo caso lanzará un AlertDialog advirtiéndolo
     * Si se ha logrado conectar no se lanzará el AlertDailog
     * Además lanzará un mensaje de Error dependiendo de si ha sido por error de conexión o porque se ha agotado el tiempo de espera de conexión
     */

    if (!isConectado && isTimeOut) {
        Handler handler = new Handler(contexto.getMainLooper());
        handler.post(new Runnable() {
            public void run() {
                Toast.makeText(contexto, "Error de conexión. No se pueden cargar los articulos", Toast.LENGTH_LONG).show();
            }
        });
    } else if (!isConectado && isIOError) {
        Handler handler = new Handler(contexto.getMainLooper());
        handler.post(new Runnable() {
            public void run() {
                Toast.makeText(contexto, "Error de conexión. No se pueden cargar los articulos", Toast.LENGTH_LONG).show();
            }
        });
    }
}

...

The call to the method:

XXXXX xxxx= new XXXXX(contexto);
xxxx.execute("","");

This is located on a button that is not clicked again until the entire process done by the AsyncTask has been completed

The code that is with XX or with rare characters is irrelevant, I have removed the excess code that is irrelevant for the case.