How to check for blank cells in datagridview

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VB

Could somebody tell me how to update table with datagridview with blank cell in it?
I have created a data entry form with datagridview with four columns in designer. I want to leave some cells of a column blank and save blank cells as zero in table. If there are no blank cells I can save datagridview content into table.

Dim nonqueryCommand As SqlCommand = thisConnection.CreateCommand()

        Try
            ' Open Connection
            thisConnection.Open()
            Console.WriteLine("Connection Opened")

            ' Create INSERT statement with named parameters
            nonqueryCommand.CommandText = _
               "INSERT  INTO Table3 (Col1, Col2,col3,col4) VALUES (@Col1, @Col2,@col3,@col4)"

            nonqueryCommand.Prepare()
        

            ' Data to be inserted
            For Each row As DataGridViewRow In DataGridView1.Rows
                If Not row.IsNewRow Then

                    nonqueryCommand.Parameters.Add("@Col1", SqlDbType.VarChar, 50).Value = DBNull.Value
                    nonqueryCommand.Parameters.Add("@Col2", SqlDbType.VarChar, 50).Value = DBNull.Value
                    nonqueryCommand.Parameters.Add("@Col3", SqlDbType.VarChar, 50).Value = DBNull.Value
                    nonqueryCommand.Parameters.Add("@Col4", SqlDbType.VarChar, 50).Value = DBNull.Value


                    nonqueryCommand.Parameters("@Col1").Value = row.Cells(1).Value.ToString()
                    nonqueryCommand.Parameters("@Col2").Value = row.Cells(2).Value.ToString()  
                    nonqueryCommand.Parameters("@Col3").Value = row.Cells(3).Value.ToString()
                    nonqueryCommand.Parameters("@Col4").Value = row.Cells(4).Value.ToString()
                    nonqueryCommand.ExecuteNonQuery()

                End If
            Next



        Catch ex As SqlException
            ' Display error
            Console.WriteLine("Error: " & ex.ToString())
        Finally
            ' Close Connection
            thisConnection.Close()
            Console.WriteLine("Connection Closed")

        End Try

What I have tried:

I don’t know if this is correct way to check for empty cell in order to save into table. I get an error when I place the code between try and Catch ex As SqlException

An OleDbParameter with ParameterName '@Col1' is not contained by this OleDbParameterCollection

If row.Cells(0).Value.ToString IsNot Nothing Then
   nonqueryCommand.Parameters("@Col1").Value = row.Cells(0).Value.ToString()
else
   nonqueryCommand.Parameters("@Col1").Value = "0"
end if