Как решить ошибку «NOT NULL constraint failed: mainapp_cartproduct.user_id»?

Всем привет! После того как добавил на сайт регистрацию стала появляться ошибка при добавлении товара в корзину IntegrityError at /add-to-cart/Interior_paints1/ NOT NULL constraint failed: mainapp_cartproduct.user_id

Ошибка появляется именно если пользователь не авторизован на сайте, если авторизоваться, то ошибка сразу же пропадает.

forms.py

class LoginForm(forms.ModelForm):

    password = forms.CharField(widget=forms.PasswordInput)

    class Meta:
        model = User
        fields = ['username', 'password']

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['username'].label = 'Логин'
        self.fields['password'].label = 'Пароль'

    def clean(self):
        username = self.cleaned_data['username']
        password = self.cleaned_data['password']
        if not User.objects.filter(username=username).exists():
            raise forms.ValidationError(f'Пользователь с логином "{username} не найден в системе')
        user = User.objects.filter(username=username).first()
        if user:
            if not user.check_password(password):
                raise forms.ValidationError("Неверный пароль")
        return self.cleaned_data


class RegistrationForm(forms.ModelForm):

    confirm_password = forms.CharField(widget=forms.PasswordInput)
    password = forms.CharField(widget=forms.PasswordInput)
    phone = forms.CharField(required=False)
    address = forms.CharField(required=False)
    email = forms.EmailField(required=True)

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['username'].label = 'Логин'
        self.fields['password'].label = 'Пароль'
        self.fields['confirm_password'].label = 'Подтвердите пароль'
        self.fields['phone'].label = 'Номер телефона'
        self.fields['first_name'].label = 'Имя'
        self.fields['last_name'].label = 'Фамилия'
        self.fields['address'].label = 'Адрес'
        self.fields['email'].label = 'Электронная почта'

    def clean_email(self):
        email = self.cleaned_data['email']
        domain = email.split('.')[-1]
        if domain in ['com', 'net']:
            raise forms.ValidationError(
                f'Регистрация для домена {domain} невозможна'
            )
        if User.objects.filter(email=email).exists():
            raise forms.ValidationError(
                f'Данный почтовый адрес уже зарегистрирован в системе'
            )
        return email

    def clean_username(self):
        username = self.cleaned_data['username']
        if User.objects.filter(username=username).exists():
            raise forms.ValidationError(
                f'Имя {username} занято'
            )
        return username

    def clean(self):
        password = self.cleaned_data['password']
        confirm_password = self.cleaned_data['confirm_password']
        if password != confirm_password:
            raise forms.ValidationError('Пароли не совпадают')
        return self.cleaned_data

    class Meta:
        model = User
        fields = ['username', 'password', 'confirm_password', 'first_name', 'last_name', 'address', 'phone', 'email']

views.py

class AddToCartView(CartMixin, View):

def get(self, request, *args, **kwargs):
    product_slug = kwargs.get('slug')
    product = Product.objects.get(slug=product_slug)
    cart_product, created = CartProduct.objects.get_or_create(
        user=self.cart.owner, cart=self.cart, product=product
    )
    if created:
        self.cart.products.add(cart_product)
    recalc_cart(self.cart)
    messages.add_message(request, messages.INFO, "Товар успешно добавлен")
    return HttpResponseRedirect('/cart/')

class LoginView(CartMixin, View):

def get(self, request, *args, **kwargs):
    form = LoginForm(request.POST or None)
    categories = Category.objects.all()
    context = {
        'form': form,
        'categories': categories,
        'cart': self.cart
    }
    return render(request, 'login.html', context)

def post(self, request, *args, **kwargs):
    form = LoginForm(request.POST or None)
    if form.is_valid():
        username = form.cleaned_data['username']
        password = form.cleaned_data['password']
        user = authenticate(
            username=username, password=password
        )
        if user:
            login(request, user)
            return HttpResponseRedirect('/')
    categories = Category.objects.all()
    context = {
        'form': form,
        'cart': self.cart,
        'categories': categories
    }
    return render(request, 'login.html', context)

class RegistrationView(CartMixin, View):

def get(self, request, *args, **kwargs):
    form = RegistrationForm(request.POST or None)
    categories = Category.objects.all()
    context = {
        'form': form,
        'categories': categories,
        'cart': self.cart
    }
    return render(request, 'registration.html', context)

def post(self, request, *args, **kwargs):
    form = RegistrationForm(request.POST or None)
    if form.is_valid():
        new_user = form.save(commit=False)
        new_user.username = form.cleaned_data['username']
        new_user.email = form.cleaned_data['email']
        new_user.first_name = form.cleaned_data['first_name']
        new_user.last_name = form.cleaned_data['last_name']
        new_user.save()
        new_user.set_password(form.cleaned_data['password'])
        new_user.save()
        Customer.objects.create(
            user=new_user,
            phone=form.cleaned_data['phone'],
            address=form.cleaned_data['address']
        )
        user = authenticate(
            username=new_user.username, password=form.cleaned_data['password']
        )
        login(request, user)
        return HttpResponseRedirect('/')
    categories = Category.objects.all()
    context = {
        'form': form,
        'categories': categories,
        'cart': self.cart
    }
    return render(request, 'registration.html', context)

Объясните пожалуйста, как мне сделать, чтобы можно было добавлять товар в корзину анонимный юзерам.