Widget Misbehaving, Shows Widget AND “default” Sidebar Contents

I am hoping this is just a WP version issue. I am working on a new theme. In the past, to widgetize a sidebar, you would add something like this right before the opening <ul>:

<?php if ( !function_exists('dynamic_sidebar')
        || !dynamic_sidebar() ) : ?>

And you’d add this right after the closing </ul>:

<?php endif; ?>

I notice now, however, that WP throws out errors when it encounters that last <?php endif; ?>. When you remove it, things settle down. Except for one thing:

As you can see from the testbed, it throws EVERYTHING into the sidebar, the original contents AND the widgets.

https://wordpress.mytestbed.com

I know this is because of that missing line of code at the end, but what is the fix? I’ve Googled and searched these forums. Any thoughts/ I wanted to widgetize this theme before releasing it, but personally, I don’t care for them so.. anyway, if anyone has any ideas, I’m all ears.

Thanks!