Clear the references to TextPropertyPanel (parent vcl::Window) held in
maCharSpacePopup and maUnderlinePopup (both are instances derived
from svx::sidebar::Popup class) via Popup::dispose()

Change-Id: Iff0c55a851d2d0ef027c356b3829b0095bc2aacb
Reviewed-on: https://gerrit.libreoffice.org/18682Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>