Programmatically changing the Title of a DeclarativeCatalogPart

I am programmatically trying to change the title of a DeclarativeCatalogPart.
The title is selected from a DropDownList when the page posts back:

Protected Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
    For Each Ele As String In aArrayList
        If DropDownList1.SelectedValue = Ele Then
            label1.Text = Ele
            DeclarativeCatalogPart1.Title = Ele
            Exit For
        End If
    Next
End Sub

label1.Text = Ele works fine.
DeclarativeCatalogPart1.Title = Ele gives the error:
System.NullReferenceException: Object reference not set to an instance of an obje ...