Behaviours timer for checking certain fields is never stopped

Some form fields cannot be monitored for changes in the way that most fields are, and so their values have to be checked at regular intervals using a function on a timer. This timer is set up when a Behaviours-applicable screen is opened, but it never torn down again after the screen is closed. Since the fields that the timed function was previously checking are no longer present in the DOM, it produces a steady stream of error messages to the browser console logs. This is not necessarily a problem in itself, but there are reports that the page becomes unresponsive if left for some time.