If you close the scene driller while the current GtWorld is highlighted, the entire UI is destroyed.

[tangentstorm]

If the scene driller has the current GtWorldElement highlighted, and you close the driller, the entire GT UI is replaced with an empty white window that cannot be closed. Possibly it just waiting for you to click the confirmation button in the now-invisible pink strip at the top.

You also get a debugger with the message "AssertionFailure: All elements must not have parent", and can recover by using the debugger to spawn a new main window by executing GtWorld open, at which point the old window becomes closable.