CascadingDropDown returning SelectedValue empty

Hello everyone.

I have a Default.aspx page which has a PlaceHolder and inside it an ascx control loaded by using the following code:

PlaceHolder.Controls.Add(LoadControl("mycontrol.ascx"));
ScriptManager.GetCurrent(this.Page).RegisterPostBackControl(PlaceHolder.Controls[0]);

Inside of mycontrol.ascx" I have insterted a button to show a ModalPopUpExtender which points to a panel and inside that panel I have an ascx control which has the CascadingDropDown controls (I know this is pretty crazy and is not the ideal but world is not ideal) so I tried to get the SelectedValue of the DropDownList and de CascadingDropDown but it always return "" or an empty string and the count of items is 1.

How I could get the correct selected value?

Ivan Dario Ospina C#.NET, T-SQL Developer