Simple DropDownList in DataGridView

I’m trying to insert values into dropdownlist that placed in datagridview - itemtemplate dynamically.
I’v typed:

 DropDownList dd=(DropDownList)gridViewRowVar.FindControl("techniciansDDL");
dd.Items.Add("ddd");

and there’s a problem: System.NullReferenceException was unhandled by user code, and something like:
The reference to the object had not been classified to the event of the object.
The error is just on runtime,
icant understand why its so complicated to insert a simpal information into ddl. plz help~!

Tell me if i wasnt clear!!!