How to Disable Textbox in Gridview If the Texbox is not null

Question

 Dear Friend ,

            I would like to Disable the Textbox in Gridview , if the textbox is not null...
I wrote the below coding in the RowEditing Event but its not working.
               

 gvAssginingView.EditIndex = e.NewEditIndex;

        TextBox TxtVal = (TextBox)gvAssginingView.Rows[e.NewEditIndex].FindControl("txtDriverCode");

        TxtVal.Enabled = false;

                  Can Anyone Guide me ...


Thanks in advance


Karthik.k