autenticación con zend

[wilmer30]

Hola:

Me está costando aprender zend, deseo realizar autenticación de usuario y el error que tengo es este

Catchable fatal error: Argument 1 passed to Zend_Auth::authenticate() must be an instance of Zend_Auth_Adapter_Interface, null given, called in C:\xampp\htdocs\biblioteca\application\controllers \LoginController.php on line 62 and defined in C:\xampp\library\Zend\Auth.php on line 115

mi controlador es este

Código PHP:

class LoginController extends Zend_Controller_Action {
    
    public function init() {
        /* Initialize action controller here */
    }
    
    public function indexAction() {
        $this->view->form = $this->getForm ();
        // $this->view->form = $this->loginAction();
    }
    public function getForm() {
        return new Application_Form_LoginForm ( array (
                'action' => '/login/process',
                'method' => 'post' 
        ) );
    }
    
    public function getAuthAdapter(array $params) {
    }
    
    public function preDispatch() {
        if (Zend_Auth::getInstance ()->hasIdentity ()) {
            // If the user is logged in, we don't want to show the login form;
            // however, the logout action should still be available
            if ('logout' != $this->getRequest ()->getActionName ()) {
                $this->_helper->redirector ( 'index', 'index' );
            }
        } else {
            // If they aren't, they can't logout, so that action should
            // redirect to the login form
            if ('logout' == $this->getRequest ()->getActionName ()) {
                $this->_helper->redirector ( 'index' );
            }
        }
    }
    public function processAction() {
        $request = $this->getRequest ();
        
        // Check if we have a POST request
        if (! $request->isPost ()) {
            return $this->_helper->redirector ( 'index' );
        }
        
        // Get our form and validate it
        $form = $this->getForm ();
        if (! $form->isValid ( $request->getPost () )) {
            // Invalid entries
            $this->view->form = $form;
            return $this->render ( 'index' ); // re-render the login form
        }
        
        // Get our authentication adapter and check credentials     
        $adapter = $this->getAuthAdapter ( $form->getValues () );
        $auth = Zend_Auth::getInstance ();
        $result = $auth->authenticate ( $adapter );
            // /
        $values = $form->getValues ();
        $adapter = new Zend_Auth_Adapter_DbTable ( $this->db );
        $adapter->setIdentity ( $values ['username'] )->setCredential ( $values ['password'] );
        $auth = Zend_Auth::getInstance ();
        $result = $auth->authenticate ( $adapter );     
        ///
        if (! $result->isValid ()) {
            // Invalid credentials
            $form->setDescription ( 'Invalid credentials provided' );
            $this->view->form = $form;
            return $this->render ( 'index' ); // re-render the login form
        }
        
        // We're authenticated! Redirect to the home page
        $this->_helper->redirector ( 'index', 'index' );
    }
    public function logoutAction() {
        Zend_Auth::getInstance ()->clearIdentity ();
        $this->_helper->redirector ( 'index' ); // back to login page
    }
}
 

por favor quisiera me expliquen el problema