Without this, vcl::Window::ImplTrackTimerHdl() will be called on a
deleted vcl::Window.

Can be reproduced with a comment having a scrollbar in a LOK client,
then clicking on the down button of the scrollbar a number of times ->
crash on exit.

Change-Id: I5d67f96e8baa199f65ec5cf39cb5d39c8162ff33
(cherry picked from commit 7c654ee9)