Not get response from server (Symfony 4)

I am trying to do login part of my project that I used symfony 4 with JWT at the back and reactjs to the front. In the postman the Api working fine and the fetch from front side has a status 200! However the response is {user: null}.

It is my code in login function:

/**
     * @Rest\Post("/login", name="api_login")
     */
    public function login()
    {

        $this->getUser();
        return $this->json([
            'user' => $this->getUser()
        ]);


    } 

Can someone guide me where i made a mistake? Thanks