Call to a member function empresas() on null - Laravel

I have two models: Empresa and Postulante. This relationship is Many-to-Many.

Empresa Model

public function postulantes()
{
    return $this->belongsToMany(Postulante::class);
}

Postulante Model

public function empresas()
{
    return $this->belongsToMany(Empresa::class)->where('empresa_postulante.activo', 1)->orderBy('empresa_postulante.created_at');
}

PostulanteController (get the Empresas for one Postulante)

public function index()
{
    $usuario_actual = \Auth::user(); //obtengo los datos del usuario en sesion
    $usuario_id = $usuario_actual->id;
    $postulante = Postulante::where('pos_usuario', $usuario_id)->first();
    $empresas = $postulante->empresas();

    return view('postulantes/dash-postulante', compact('empresas'));
}

As there is no data in the relationship table (empresa_postulante), I get the following error message:

Call to a member function empresas() on null

via Chebli Mohamed