Error : object reference not set an inestance of an object

I want to get data from dropdownist to textbox but
when I put first character in textbox give me first  data  start this character  from dropdownlist to textbox but another datat start this character can't show in textbox and show this Error :
object reference not set of an inestance of an object
Protected Sub TxtName_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles TxtName.TextChanged Dim c As New Company c.namepr = TxtName.Text Dim dt As New DataTable dt = c.search(Request.QueryString("idvar")) DDLComp.DataSource = dt<br> DDLComp.DataTextField = &quot;comp-name&quot DDLComp.DataValueField = &quot;comp_CompanyID DDLComp.DataBind()<br> txtname.text=ddlcomp.selecteditem.text End Sub  
please help me