balancebox, fix unhidden overlay

• hide loading overlay before returning from initMap if pendingObjects == 0 (thanks JonnyJ for reporting)

• always hide loading overlay onStop

• fix corner case for >1 empty level at the end of the user level: following use-case: start new level in editor -> change something, don't save -> click next -> click "yes" continue -> new empty level is started -> iterate -> >1 empty levels in the users file