A DataGrid displays 3 columns from a database table - ID, Name &
Subject. When the DataGrid is in the editable mode, I want the 3rd
column Subject to a DropDownList so that users can change the subject.
This is how I tried it:

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<script runat="server">
   Public dSet As DataSet
   Public strSQL As String
   Public sqlDapter As SqlDataAdapter
   Public sqlConn As New SqlConnection(".......")

   Sub Page_Load(ByVal obj As Object, ByVal ea As EventArgs)
       strSQL = "SELECT * FROM tblSS"
       Call LoadData(strSQL)

       If Not (Page.IsPostBack) Then
           dgSS.DataBind()
       End If
   End Sub

   Sub LoadData(ByVal SQLQuery As String)
       sqlDapter = New SqlDataAdapter(strSQL, sqlConn)

       dSet = New DataSet
       sqlDapter.Fill(dSet, "SS")

       dgSS.DataSource = dSet.Tables("SS").DefaultView
   End Sub

   Sub Edit_Command(ByVal obj As Object, ByVal ea As
DataGridCommandEventArgs)
       If (ea.Item.ItemType = ListItemType.Item Or ea.Item.ItemType =
ListItemType.AlternatingItem) Then
           Dim ddl As DropDownList

           ddl = CType(ea.Item.FindControl("ddlSubject"),
DropDownList)

           strSQL = "SELECT DISTINCT(Subject) FROM tblSS"
           sqlDapter = New SqlDataAdapter(strSQL, sqlConn)

           dSet = New DataSet
           sqlDapter.Fill(dSet, "Subject")

           ddl.DataSource = dSet.Tables("Subject")
           ddl.DataBind()
       End If

       dgSS.EditItemIndex = ea.Item.ItemIndex

       strSQL = "SELECT * FROM tblSS"
       Call LoadData(strSQL)
       dgSS.DataBind()
End Sub
</script>

<form runat="server">
<asp:DataGrid ID="dgSS" OnEditCommand="Edit_Command" runat="server">
<Columns>
<asp:TemplateColumn HeaderText="#">
<ItemTemplate>
<asp:Label ID="lblID" Text='<%# Container.DataItem("ID") %>'
runat="server"/>.
</ItemTemplate>
</asp:TemplateColumn>

<asp:BoundColumn DataField="SName" HeaderText="NAME"/>

<asp:TemplateColumn HeaderText="SUBJECT">
<ItemTemplate>
<asp:Label ID="lblSubject" Text='<%# Container.DataItem("Subject") %>'
runat="server"/>
</ItemTemplate>
<EditItemTemplate>
<asp:DropDownList ID="ddlSubject" DataTextField="Subject"
runat="server"/>
</EditItemTemplate>
</asp:TemplateColumn>
<asp:EditCommandColumn CancelText="CANCEL" EditText="EDIT"
HeaderText="EDIT" UpdateText="UPDATE"/>
</Columns>
</asp:DataGrid>
</form>
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But when I click the EDIT link in the DataGrid to change the DataGrid
into editable mode, the following error gets generated:

Object reference not set to an instance of an object.

pointing to the ddl.DataSource...... line in the above code.

What am I doing wrong?