Fix NPE in allowOverlappingTransitions

The OneShotPreDrawListener from triggerViewsReady() ->
sendSharedElementDestination() may be called after clearState() if
the activity has stopped before allowing to draw. Then it will cause
NPE because clearState() set mWindow to null.

Bug: 306974798
Test: Use scene transition and stop draw by pre-draw listener.
      Then allow to draw after activity is stopped.
Change-Id: I3a714f97a6abcee51ca004ca3c919250c5d5257f