I tested the same scenario locally and I was not able to reproduce any issue.
I filled RadGridView with data from the Customers table in the Northwind database after I had set some of the ContactName's values to be NULL. Then I filtered on the null values and it worked fine. I also tried filtering on the values in the Region column.
I have attached my test solution as a reference. You just need to connect to the Northwind database. Then, the Region column has some NULL values to filter on.
May I ask you to modify my solution so that I can observe the problem?
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