Cakephp 3 Error: Call to a member function identify() on a non-object

Question

I am new to cakephp and started learning cakephp3.2 directly I have created a function in my custom controller as follows.

public function login(){
    if($this->request->is('post')){

        $user = $this->auth->identify();

        if($user){
            $this->Auth->setUser($user);
            return $this->redirect($this->Auth->redirectUrl());
        }else{
            $this->Flash->error(__('Username or password is incorrect'), [
            'key' => 'auth'
        ]);
        }

    }
}


class AppController extends Controller
{

public function initialize()
{
    parent::initialize();
    $this->loadComponent('RequestHandler');
    $this->loadComponent('Flash');

    $this->loadComponent('Auth',[
        'authenticate' => [
            'Form' => [
                'fields' => [
                    'username' => 'email',
                    'password' => 'password'
                ]
            ]
        ],
        'authError' => 'Did you really think you are allowed to see that?',
        'loginAction' => [
            'controller' => 'Users',
            'action' => 'login'
        ],
        'storage'=>'Session'
    ]);

    $this->Auth->allow(['display']);
}

public function beforeRender(Event $event)
{
    if (!array_key_exists('_serialize', $this->viewVars) &&
        in_array($this->response->type(), ['application/json', 'application/xml'])
    ) {
        $this->set('_serialize', true);
    }
}

}

I don't understand how to deal with it.Should I load any external component or helper ?