填充1列VB.Net MySQL

Question

当填充DataGridView时，我想得到All ItemId并创建一个公共子，它可以汇总7个表中的所有量化并需要一个ItemId。

本守则是Public Sub

           For Each row As DataGridViewRow In dgvStocks.Rows
                For index As Integer = 0 To dgvStocks.RowCount - 1
                    itemid = CStr(row.Cells(0).FormattedValue)              
                    With cmd
                        .Connection = con
                        .CommandText = cmdString 
                        'string query that can sum all the qty from another tables that required all ItemId
                        .Parameters.Clear()
                        .Parameters.AddWithValue("@ItemId", itemid)
                    End With

                    dgvStocks.Rows(index).Cells(3).Value = Convert.ToString(cmd.ExecuteScalar())

                Next
            Next

现在我收到了错误Object reference not set to an instance of an object.

导致此错误的原因是什么？如何在没有该错误的情况下运行此子程序并使其正常工作？