[android] "android.view.WindowManager$BadTokenException: Unable to add window" on buider.show()

Try this :

    public class <class> extends Activity{

    private AlertDialog.Builder builder;

    public void onCreate(Bundle savedInstanceState) {
                    this.requestWindowFeature(Window.FEATURE_NO_TITLE);
                    super.onCreate(savedInstanceState);

                setContentView(R.layout.<view>); 

                builder = new AlertDialog.Builder(<class>.this);
                builder.setCancelable(true);
                builder.setMessage(<message>);
                builder.setInverseBackgroundForced(true);

        //call the <className> class to execute
}

    private class <className> extends AsyncTask<String, Void, String>{

    protected String doInBackground(String... params) {

    }
    protected void onPostExecute(String result){
        if(page.contains("error")) //when not subscribed
        {   
           if(builder!=null){
                builder.setNeutralButton("Ok",new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int whichButton){
                    dialog.dismiss();
                        if(!<condition>)
                        {
                        try
                        {
                        String pl = ""; 

                        mHelper.<flow>(<class>.this, SKU, RC_REQUEST, 
                        <listener>, pl);
                        }

                        catch(Exception e)
                        {
                        e.printStackTrace();
                        }
                    }  
                }
            });

            builder.show();
        }
    }

}
}