Error for a dynamic, Dim filenameDDL As String = DDL.SelectedItem.Text, ---System.NullReferenceException: Object reference not set to an instance of an object.

For a dynamic DropdownList DDL, I coded,
            Dim filenameDDL As String = DDL.SelectedItem.Text
to collect the SelectedItem from the DDL.  The following error msg popped up:

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
I have successfully used the similar code for a Design DropdownList1. 
What does the error msg mean?  How to fix it?



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            Dim DDL = New DropDownList()
            DDL.ID = "ContentsDDL" & ID
  ...