Nop 4.2: Error when set shipping is not required

Hi,

After I disabled "Shipping enabled" option, when I added this product to shipping cart and then I tried to checkout,  I'm using one page check out btw, in the step 4 Confirm order, it will show an error "Shipping total couldn't be calculated".
I have two shipping providers, one is Shipping.FixedByWeightByTotal and the other is a plugin, and both are enabled.
I have checked source code and noticed the program will eventually goes to

if (!shippingTotal.HasValue)
                return null;

I'm kinda confused: if I don't have shipping enabled, then I won't have shippingTotal, shippingTotal will be null, but if shippingTotal is null, the program will throw an exception "Shipping total couldn't be calculated".
Am I wrong?
Anyone knows how to disable shipping for a specific product?

Thanks.

Hi
I have the same issue till me if it solved for you

Unclicking Shipping Enabled on a Product
Restart the web - maybe the setting has been cached or not updated ?
Should be saying Shipping: Not required in the check out totals ?

If you still have the problem after restarting the site, then when you see the warning message, immediately check the System Log for any error.