how to set the vale in Drop down in gridview template Colums

I am Using Gridview with template field of dropdown.


In databound event First I Populate template dropdown and then i set its value by using

gvReading_ddlgv.Items.FindByValue(strLastReading).Selected = True

It gived me error at the time of select value


"Object reference not set to an instance of an object."

although the data which i try to set in dropdown its present becuase first i
populate the dropdown and then set the value by using