使用AsyncTask获取响应时出错？

Question

这是我的代码，我尝试登录我使用异步任务来完成任务。

MainActivity extends Activity {

//define controls
EditText  txt_uname, txt_pwd;
TextView txt_Error;
Button btnLogin;
String response = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //initalise controls
    txt_uname=(EditText)findViewById(R.id.txtUsername);
    txt_pwd=(EditText)findViewById(R.id.txtPassword);
    btnLogin =(Button)findViewById(R.id.btnLogin);
    txt_Error =(TextView)findViewById(R.id.txtError);

    btnLogin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub

            String uname = txt_uname.getText().toString();
            String pwd = txt_pwd.getText().toString();

            validateUserTask task = new validateUserTask();
            task.execute(new String[]{uname, pwd});
        }
    }); //close on listener
}// close onCreate

private class validateUserTask extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... params) {
        // TODO Auto-generated method stub
        ArrayList<NameValuePair> postParameters = new ArrayList<NameValuePair>();
        postParameters.add(new BasicNameValuePair("username", params[0] ));
        postParameters.add(new BasicNameValuePair("password", params[1] ));
        String res = null;
        try {
            response = CustomHttpClient.executeHttpPost("https://xyz/restapi/login", postParameters);
            res=response.toString();
            res= res.replaceAll("\\s+","");
        }
        catch (Exception e) {
            txt_Error.setText(e.toString());
        }
        return res;
    }//close doInBackground

    @Override
    protected void onPostExecute(String result) {
        if(result.equals("1")){
            //navigate to Main Menu
            Intent i = new Intent(MainActivity.this, MainMenuActivity.class);
            startActivity(i);
        }
        else{
            txt_Error.setText("Sorry!! Incorrect Username or Password");
        }
    }
}

}

我没有得到其他API的响应，但是当我在Postman中测试时它工作正常。任何人都可以让我知道我错过了什么，让我知道是否有任何教程吗？