Otherwise it was disposed in the middle of its own mouse event handler
FuSelection::MouseButtonUp().

Change-Id: I0d96048526dece7232e5a777364ad4da1d64c904
Reviewed-on: https://gerrit.libreoffice.org/37976Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>