In all menu screens, except the realms screen, it is possible to press "Back" or "Done" to navigate to any other menu screen, or back to the title.

However, upon clicking the "Realms" button, you are stuck, unless you know where to find the ESC key. A true escape room of sorts.

However, upon closer inspection, it seems there is a back button. It just doesn't appear, at least not when you don't have any realms. This appears for a slither of a second when exiting from the 'Invitations' screen. (I had to record a video and cut the frame out to demonstrate)

I would think that the Back button should always be present, not just when you have a realm.