crashreport: 644837b5-c445-4779-a75d-dd69fc2e3a6f

This can be reproduced by switching to master slide sidebar in impress
, use the mouse to wheel up and down and then go straight to clicking
the window manager close button

The dispose doesn't ultimately drop its reference to the document
so keeps it open if the scroll-wheel previous event widget vclptr
keeps the widget around

Change-Id: Ic90bcadf2489094358737be7f522129bd555a1ba

sd/source/ui/sidebar/MasterPagesSelector.cxx

@@ -112,6 +112,7 @@ void MasterPagesSelector::dispose()
 
     Link<MasterPageContainerChangeEvent&,void> aChangeListener (LINK(this,MasterPagesSelector,ContainerChangeListener));
     mpContainer->RemoveChangeListener(aChangeListener);
+    mpContainer.reset();
     PreviewValueSet::dispose();
 }