Unit test with BelongsTo return null

Hi,

I'm trying to make a Unit Test on my service, I have a method in my service class who is doing this :

private function generateTitle(): string
    {
        if ($this->forSecretary) {
            return sprintf(
                'RESERVATION MOTOBLEU Course n°%s - %s / %s',
                $this->reservation->reference,
                ucfirst($this->reservation->entreprise->nom),
                $this->reservation->passager->nom
            );
        } else {
            $piloteLabel = $this->generatePiloteLabel();
            return sprintf(
                'Course n°%s - %s / %s / %s',
                $this->reservation->reference,
                ucfirst($this->reservation->entreprise->nom),
                $this->reservation->passager->nom,
                'plt: '. $piloteLabel
            );
        }
    }

I would like to know how to mock the $this->reservation->entreprise->nom and $this->reservation->passager->nom both are in my Reservation model like so :

public function entreprise(): BelongsTo
    {
        return $this->belongsTo(Entreprise::class);
    }

public function passager()
    {
        return $this->belongsTo(Passager::class);
    }

I tried this but I have ErrorException : Attempt to read property "nom" on null :

protected function setUp(): void
    {
        parent::setUp();

        $this->reservation = $this->createMock(Reservation::class);
        $this->reservation->is_cancel = false;

        $this->pilote = $this->createMock(Pilote::class);
        $this->pilote->full_name = "test";

        $this->entreprise = $this->createMock(Entreprise::class);
        $this->entreprise->nom = "test";

        $this->passager = $this->createMock(Passager::class);
        $this->passager->nom = "test";

        $this->reservation->method("pilote")->willReturn($this->pilote);
        $this->reservation->passager = $this->passager;
        $this->reservation->entreprise = $this->entreprise;



        $this->calendarService = new GoogleCalendarService();
    }

Thanks for helping.