NOP Import Problems

I have set up Nop 2.2 locally. I am having trouble with the import  from excel feature.
1. If I download the existing catalog and reimport it with out making any changes everything works fine, except it continues to remport the existing products all over again.
2. When I add new products to the spreadsheet and make sure all the columns are filled in like the existing products I am getting the following error. Object cannot be cast from DBNull to other types.
I have deleted all the rows below, I have copied the data to a new spreadsheet, but I am still getting the same error.

Any Ideas
Thanks Matt

1. In order to prevent adding of the new products instead of updating the existing one just set unique SKU for each product

Ok, That is easy enough.

What do you think about the error.Object cannot be cast from DBNull to other types

https://www.nopcommerce.com/boards/search.aspx?searchterms=object+cannot+be+cast+from+dbnull+to+other+types

Got it.
This is a required field "RequiredProductVariantIds"