We are aware of the problem, and it is on our list of things that need to be fixed. Until we can fix the problem, a workaround is to do the following:
- Open either the in-game Store or the Options menu (the orange gear icon in the corner).
- Close whichever window you opened in Step 1.
- Press the Back button, which should now be working again. This should bring up the dialog box where you can confirm that you want to exit the game.