How to use yii2-user as sub-module inside another module - module

Question

I want to implement one module in an existing application.In that module I am trying to use yii2-user module.The yii2 docs say we can do that.
namespace app\modules\forum;
class Module extends \yii\base\Module
{
public function init()
{
parent::init();
$this->modules = [
'admin' => [
// you should consider using a shorter namespace here!
'class' => 'app\modules\forum\modules\admin\Module',
],
];
}
}
I have module code as
namespace app\modules\cdas;
class cdas extends \yii\base\Module
{
public $controllerNamespace = 'app\modules\cdas\controllers';
public function init()
{
parent::init();
// custom initialization code goes here
$this->modules = [
'user' => [
'class' => 'dektrium\user\Module',
'modelMap' => [
'Profile' => 'app\modules\cdas\models\users\Profile',
'User'=>'aapp\modules\cdas\models\users\User',
],
'controllerMap' => [
'settings' => 'app\modules\cdas\controllers\user\SettingsController',
'admin' => 'app\modules\cdas\controllers\user\AdminController',
'role' => 'app\modules\cdas\controllers\user\RoleController',
'security' => 'app\modules\cdas\controllers\user\SecurityController',
],
];
}
}
But when I use the above methodology and try to access
http://localhost/<app_name>/web/cdas/user/security/login
I get the following error
PHP Notice – yii\base\ErrorException
Trying to get property of non-object
in ....vendor\dektrium\yii2-user\views\_alert.php
/**
* #var $module dektrium\user\Module
*/
<?php if ($module->enableFlashMessages): ?>
Please suggest proper way to implement the sub-module.