Dispalying error Column ID Does not allow null values

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C#

ASP.NET

    private void AddNewRow()
    {
       
        int rowIndex = 0;
        if (ViewState["table1"] != null)
        {
            DataTable dtCurrentTable = (DataTable)ViewState["table1"];
            DataRow drCurrentRow = null;
            if (dtCurrentTable.Rows.Count > 0)
            {
                for (int i = 1; i <= dtCurrentTable.Rows.Count; i++)
                {

             oreach (System.Data.DataColumn col in dtCurrentTable.Columns) col.ReadOnly = false; 
                    DropDownList ddltype = (DropDownList)gvFormula.Rows[rowIndex].Cells[1].FindControl("ddltype");
                    DropDownList ddlproduct = (DropDownList)gvFormula.Rows[rowIndex].Cells[2].FindControl("ddlproduct");
                    TextBox txtwt = (TextBox)gvFormula.Rows[rowIndex].Cells[3].FindControl("txtWeight");


                    drCurrentRow = dtCurrentTable.NewRow();
                    drCurrentRow["RowNumber"] = i + 1;                    
                    dtCurrentTable.Rows[i - 1]["product"] = ddlproduct.SelectedItem.Text;
                    dtCurrentTable.Rows[i - 1]["weight"]= txtwt.Text;
                    rowIndex++;

                }
               
                dtCurrentTable.Rows.Add(drCurrentRow);

//Error  " Column 'Product' does not allow nulls."

                ViewState["table1"] = dtCurrentTable;
                gvFormula.DataSource = dtCurrentTable;
                gvFormula.DataBind();
               
            }
        }
        else
        {
            Response.Write("ViewState is null");
        }
        SetPreviousData();
    }

I have gridview when I click add button Iam calling this method . the agenda of this method is to add the new row > But I am Getting the error " Column 'Product' does not allow nulls."