Closing app by pressing back button is disabled in Unity by default and enabling this feature – totally bad idea, because when you play in games with on-screen controls, you necessarily will click on back button while playing.
In addition let me explain what i see:
1) App is opened and web-site is loaded.
2) I pressed back button and have a white screen.
3) I pressed back button again and app is closed.
It’s not good and checking web browser history is a point, where you need to dig to avoid this behavior of “null”-page.