On the main screen the tab bar and app bar are fixed. Only the content changes by selecing the tab. So now there is no need to use the back button. Only the screens "ExhibitionScreen" and "AudioGuideScreen" are pushed to the stack and therefore have a button to go back. Added a new Widget MapPopUpScreen to open the map as an pop-up screen. You now cannot get back to the introduction screen from the main screen.