Как включить текстовое поле, вложенное в gridview, в событии rowediting ?

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Как включить текстовое поле, вложенное в Gridview, в событии RowEditing ?

protected void MyGrid_RowEditing(object sender, GridViewEditEventArgs e)
        {
            string CampID;
            //int id = Convert.ToInt32(MyGrid.DataKeys[e.].ToString());
            CampID = (Session["CampaignID"].ToString());
            mf.UpdateStatus(CampID, 'X');
            MyGrid.EditIndex = e.NewEditIndex;
            //string lblOfferType = MyGrid.Rows[e.NewEditIndex].Cells[4].Text;
            GridViewRow currentRow = this.MyGrid.Rows[e.NewEditIndex];

            string lblOfferType = (currentRow.FindControl("lblOffer_Type") as Label).Text;
            //string freeQty = (currentRow.FindControl("txtFree_Qty") as TextBox).Text;
            //string discPercent = (currentRow.FindControl("txtDisc_Percent") as TextBox).Text;
            //TextBox discValue = (currentRow.FindControl("txtDisc_Value") as TextBox);
            //string offerPrice = (currentRow.FindControl("txtOffer_Price") as TextBox).Text;
            //string freeArticle = (currentRow.FindControl("txtFree_Article") as TextBox).Text;

            if (lblOfferType == "VALUE DISCOUNT")
            {
                GridViewRow row = (GridViewRow)((Control)sender).NamingContainer;
                GridViewRow currentRow1 = this.MyGrid.Rows[e.NewEditIndex];
                ((TextBox)MyGrid.Rows[e.NewEditIndex].FindControl("txtDisc_Value")).Enabled = false;
                //discValue.Enabled = true;
                //(currentRow.FindControl("txtDisc_Value") as TextBox).Enabled = true;
                //((TextBox)currentRow.FindControl("txtDisc_Value")).Enabled = true;
            }
}

Ошибка: ссылка на объект не установлена на экземпляр.

Пожалуйста, предоставьте решение.

Что я уже пробовал:

protected void MyGrid_RowEditing(object sender, GridViewEditEventArgs e)
        {
            string CampID;
            //int id = Convert.ToInt32(MyGrid.DataKeys[e.].ToString());
            CampID = (Session["CampaignID"].ToString());
            mf.UpdateStatus(CampID, 'X');
            MyGrid.EditIndex = e.NewEditIndex;
            //string lblOfferType = MyGrid.Rows[e.NewEditIndex].Cells[4].Text;
            GridViewRow currentRow = this.MyGrid.Rows[e.NewEditIndex];

            string lblOfferType = (currentRow.FindControl("lblOffer_Type") as Label).Text;
            //string freeQty = (currentRow.FindControl("txtFree_Qty") as TextBox).Text;
            //string discPercent = (currentRow.FindControl("txtDisc_Percent") as TextBox).Text;
            //TextBox discValue = (currentRow.FindControl("txtDisc_Value") as TextBox);
            //string offerPrice = (currentRow.FindControl("txtOffer_Price") as TextBox).Text;
            //string freeArticle = (currentRow.FindControl("txtFree_Article") as TextBox).Text;

            if (lblOfferType == "VALUE DISCOUNT")
            {
                GridViewRow row = (GridViewRow)((Control)sender).NamingContainer;
                GridViewRow currentRow1 = this.MyGrid.Rows[e.NewEditIndex];
                ((TextBox)MyGrid.Rows[e.NewEditIndex].FindControl("txtDisc_Value")).Enabled = false;
                //discValue.Enabled = true;
                //(currentRow.FindControl("txtDisc_Value") as TextBox).Enabled = true;
                //((TextBox)currentRow.FindControl("txtDisc_Value")).Enabled = true;
            }
}

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