Select window can put the focus on a specific window by name, so your script can use that button to go back to bookings. You describe a system of three windows so it is not 100% certain to me when I read this description that you really only have one window open at the time you get this error message.
But there is a known issue that can cause this error to appear even though you only have one window open when using FileMaker Go on an iOS device. The immediate fix is to double click the home button and "flick" the filemaker go app up to quit the app. Then you can restart FM GO and re-open the file.
It's also possible to create a script that checks for this issue and if it detects the problem, a message appears telling the user to restart FM GO. (Hate to do that, but it beats having frustrated users.)