Call to a member function getFirstRow() on bool

I am converting from codeigniter 3 to 4. I would like to use the find function, but I get Call to a member function getFirstRow () on bool

Model

PHP Code:

namespace App\Models;

use CodeIgniter\Model;
use App\Entities\Studente_e;

class Studente_m extends Model
{
protected $table      = 'tblstudenti';
protected $primaryKey = 'NUM_MATRIC';
protected $useAutoIncrement = true;
protected $returnType = 'App\Entities\Studente_e'; // configure entity to studebnte
protected $useSoftDeletes = true;

protected $allowedFields = ['name', 'email'];

/*protected $useTimestamps = false;
protected $createdField  = 'created_at';
protected $updatedField  = 'updated_at';
protected $deletedField  = 'deleted_at';*/

protected $validationRules    = [];
protected $validationMessages = [];
protected $skipValidation    = false;
protected $useTimestamps = true;
function verifica_stud($id, $email){
return $result = $stud_m->find($id, $email);
}} 


Controller

PHP Code:

namespace App\Controllers;
use CodeIgniter\Controller;
use App\Models\login_m;
use App\Models\studente_m;
use App\Models\professore_m;

use App\Libraries\MY_Form_validation;

class Registra extends BaseController
{
public function __construct()
{...
$this->studente_m= new studente_m();
...}
public function index()
{...
$query = $this->studente_m->verifica_stud($id, $email);
...}