Just a follow-up (esp. anyone reading this later) ... Thanks to the tips from Valentin (and some poking around) I figured out how to get into the app. and get it working.

The link on the app button was: /web#menu_id=728&action_id=3
I changed that to: /web#menu_id=729&action_id=3
(changed the 728 to 729)
And the app opened up.

Then I could open up the app in studio.

Then I edited the menu.

The root menu had no action associated with it.

So, as Valentin suggested,
I changed the action to "ir.actions.act_window" and then selected the first sub menu item (in the 2nd dropdown of the Action field - which isn't all that obvious at first) that linked to a data view, then everything worked properly.

Big Thanks to @Valentin for sharing this. Hope all this helps someone else resolve a similar issue.