[TIMOB-4007] Closing a window in a nav group doesn't destroy the window

Scenario: Inside a window that was opened in a tabgroup - when clicking the back button, the window closes but there is no easy way to set the closed window to null. In this context, when the back button UI is generated automatically, the platform should handle freeing up the resources for the developer. Attached is an examples that tries to mimic a customer's app as much as possible, although it's not an ideal way of doing this, it demonstrates the point..