How I enable textbox nested inside gridview in rowediting ?

Question

How can i enable textbox nested inside Gridview in RowEditing event ?

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;
}
}


Error: Object reference not set to an instance.