Error when importing products — "Object reference not set to an instance of an object."!

Hello friends!

We decided to upgrade our nopCommerce to a latest one.

So, as we have been using nopCommerce 3.50 and in order to avoid different kinds of errors during upgrading, we decided just to install a new version and import all categories, products etc. to a new site.

We have already imported about 300 products and it was importing fine with no erros.

But now (unexpectedly) it gives this error — "Object reference not set to an instance of an object."!

I have searched for some solution, but couldn't find what to do in this specific situation...

I have exported different categories of products from old site and tried to import each of them to the new site... In any event it gives the same error.

I went to the logs... and saw the following...

Log level  
Error
Short message  
Object reference not set to an instance of an object.
Full message  
System.NullReferenceException: Object reference not set to an instance of an object. at Nop.Services.ExportImport.ImportManager.ImportCategoriesFromXlsx(Stream stream) at Nop.Admin.Controllers.CategoryController.ImportFromXlsx()
IP address  
188.***.**.58
Customer  [email protected]
Page URL  
http://ali.ru/admin/category/importfromxlsx
Referrer URL  
/admin/category/list
Created on  
6/9/2017 4:42:49 AM

So, how to fix it? What to do in order to get rid of these errors?

Thanks for your help in advance!

Do you get the error when importing Categories, or also when importing Products?
Importing from another version does not always work.  Export from your new version to see the format of the file.  (Convert the old file to match columns, etc.)

New York wrote:

It was working just fine before... I have already imported 300 products and it worked with no errors. I exported excel file from the same old site, everything was okay, but now somehow it gives that error...(

Sorry, guys!

It was my silly mistake...

I was trying to import products in categories...

Sorry again :)))