Erro ao persistir dados

Call to a member function persist() on null

Meu controlador:

 public function handle(Request $request): Response
    {
        $body = $request->getContent();
        $data = json_decode($body);

        $medico = new Medico();
        $medico->setNome($data->nome);
        $medico->setCrm($data->crm);

        $this->em->persist($medico);
        $this->em->flush();

        return new JsonResponse($medico->extract());
    }

Meu model:

<?php

namespace App\Entity;

class Medico
{
    public int $crm;
    public string $nome;

    function getCrm(): int
    {
        return $this->crm;
    }

    function setCrm($crm): self
    {
        $this->crm = $crm;
        return $this;
    }

    function getNome(): string
    {
        return $this->nome;
    }

    function setNome($nome): self
    {
        $this->nome = $nome;
        return $this;
    }

    public function extract()
    {
        return get_object_vars($this);
    }
}