Object reference not set to an instance of an object.

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C#

C#4.0

ASP.NET4

I've use this code for fill value in drop down which is used in grid view but its generate an error "Object reference not set to an instance of an object." on the first line


protected void lov_room_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList lov_gvrack = (DropDownList)GridView1.FooterRow.FindControl("lov_gvrack");
string str = @"SELECT rack_no
FROM S01_M03_SpaceManagment_Rack
WHERE (room_no = '" + lov_room.SelectedValue.Trim() + "')";
cnn.DropDown3(lov_gvrack, str, "rack_no", "rack_no");
}



please any body solve my problem...
Thanks.