Mixing Tabbed Pages and Content Pages with Login and Logout

Hi,

I am new to Xamarin (and coding in general, but learning...) and have a question about switching pages.

I have a Login page which is called on Startup This is a ContentPage.

**Login.xaml **

Upon successful login it fires this. I don't want the MainPage to have a "Back" button to go back to the login screen

Login.xaml.cs

MainPage.xaml is a TabbedPage with Navigation

Each of the pages there are all Valid. The Settings page has a Logout button.
**
Settings.xaml**

With the Logout in Settings.xaml.cs doing this

But the end result is that I have the Login screen... with the Tab bar at the bottom

What have I done wrong? I'd like it so when I logout the tabs aren't visible.

Thanks,
Matt