Prevent NPE when dragging last bubble from expanded state

- We null out stackview when it's removed from the window, this happens
  when the last bubble is removed from the view
- When the last bubble is dragged out, we trigger some other updates
  these get applied *after* the remove step so stackview is null at that
  time. These updates are to set selected bubble to null & collapse the
  stack. These don't technically need to tell stackView about them.
- I don't think this is an ideal solution

Test: drag out last bubble while stack is expanded
Fixes: 157778325
Change-Id: Ia55bf3ba315599ed8afb390ad481e0d3700b1b97