I finally got it to work.
As you know, when the control is initially created, Visual Studio auto generates the following code:
<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="DemoControl.ascx.cs" Inherits="SitefinityWebApp.MyControls.DemoControl" %>
I took this code out and now the custom controls works fine.
Not sure if this was the correct method to make it work given that the training video
shows the code still in the control, but it does work now.