'NoneType' object has no attribute

class CartItem(models.Model):
    product = models.ForeignKey(Product)
    qty = models.PositiveIntegerField(default=1)
    item_total = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
 
    def __unicode__(self):
        return unicode(self.product.shown_name)
 
 
class Cart(models.Model):
    items = models.ManyToManyField(CartItem, blank=True)
    cart_total = models.DecimalField(max_digits=9, decimal_places=2, default=0.00)
 
    def __unicode__(self):
        return unicode(self.id)

def cart_view(request):
    cart = Cart.objects.first()
    context = {
        'cart': cart
    }
    return render(request, 'products/cart.html', context)
 
def add_to_cart_view(request, product_slug):
    print(1)
    product = Product.objects.get(slug=product_slug)
    new_item, _ = CartItem.objects.get_or_create(product=product, item_total=product.default_price)
 
    cart = Cart.objects.first()
    print(len(Cart.objects.all()))
    print(dir(cart))
 
    if new_item.product not in cart.items.all():
        cart.items.add(new_item)
        cart.save()
        print(cart.items.all())
        return HttpResponseRedirect('/cart/')
    else:
        return HttpResponseRedirect('/cart/')

url(r'^add_to_cart/(?P<product_slug>[-\w]+)$', add_to_cart_view, name='add_to_cart'),
url(r'^cart/$', cart_view, name='cart'),

<a href='{% url "add_to_cart" product_slug=product_item.slug %}'><button class="btn btn-primary" type="submit">В корзину</button></a>

return HttpResponseRedirect('/cart/')

return HttpResponseRedirect('/cart/')