Trying alter Parking demo

I have the Parking demo running, both on Chrome desktop and chrome mobile.

As a learning exercise, I decided to add a panel to the MainTabsheet; I copied the ShiftsView code to another class, modified all the names to display a list of people instead of a list of shifts, and added "AddTab(new PeopleView(), “peopletab”, “People”) to the constructor for MainTabSheet.

The result is that my additional tab shows up, but the ShiftsView disappears. If I remove the addition to ShiftsView from the code, the space for its icon still appears at the bottom of the app, but there’s no text or icon and clicking on it has no visible effect.

Can someone explain this behavior? I was expecting the additional tab to show up if I added it, but that it would not have an effect on the other tabs in MainTabsheet/TabBarView.