prevent crash when trying to edit broken chart documents

Change-Id: Ie5fb706011ca600b42e5e2d3770e919b56347edd
Reviewed-on: https://gerrit.libreoffice.org/46938Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Jenkins <ci@libreoffice.org>
(cherry picked from commit 1d2eb0b6)
Reviewed-on: https://gerrit.libreoffice.org/47004Reviewed-by: Michael Stahl <mstahl@redhat.com>

chart2/source/controller/sidebar/ChartElementsPanel.cxx

@@ -397,10 +397,16 @@ css::uno::Reference<css::chart2::XChartType> getChartType(const css::uno::Refere
 
     css::uno::Sequence<css::uno::Reference<css::chart2::XCoordinateSystem>> xCooSysSequence(xCooSysContainer->getCoordinateSystems());
 
+    if (xCooSysSequence.getLength() == 0)
+        return css::uno::Reference<css::chart2::XChartType>();
+
     css::uno::Reference<css::chart2::XChartTypeContainer> xChartTypeContainer(xCooSysSequence[0], css::uno::UNO_QUERY_THROW);
 
     css::uno::Sequence<css::uno::Reference<css::chart2::XChartType>> xChartTypeSequence(xChartTypeContainer->getChartTypes());
 
+    if (xChartTypeSequence.getLength() == 0)
+        return css::uno::Reference<css::chart2::XChartType>();
+
     return xChartTypeSequence[0];
 }