following up on 8b5f8c26 "Avoid crash with empty
Chart document", when you then close the Writer doc (discarding the changes), it
crashes in a chart::sidebar::ChartSidebarModifyListener::disposing call whose
mpParent (that was a ChartElementsPanel) is already destroyed.  The problem is
that the ChartElementsPanel ctor creates an mxListener pointing to this, then
calls Initialize, which calls xBroadcaster->addModifyListener(mxListener) and
then, during updateData, throws a RuntimeException when xDiagram is null in
getChartType, causing the ChartElementsPanel object to immediately be destroyed,
but the listener still registered.  This apparently needs fixing independently
of the immediate problem.  (The immediate problem is a regression introduced
with d7748842 "hide grid and axes options in
chart elements if not supported, tdf#94297".)

Change-Id: I2fe4f2a59600664f6210456c65e033f9453f4cc9
(cherry picked from commit 429e4dc6)
Reviewed-on: https://gerrit.libreoffice.org/30146Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>