Gio's GioView.onDestroyView sends ViewEvent{View: 0} as its detach
signal when the view is destroyed — i.e. the activity going away on a
recents-wipe. An app that forwards the event to Enable() passes a zero
view through to registerFragment, whose GetObjectClass(null) aborts the
process (JNI DETECTED ERROR: java_object == null in call to
GetObjectClass).
Enable() now returns early on a zero view so all callers are covered
even if they forward the event unfiltered, and registerFragment
null-checks as defense in depth. Same fix as pad (fdbffc9); verified
there on a Pixel 9 Pro / Android 17 where the recents swipe used to
crash the app.
