there's a bug that I had copied: update() checks each list node: if it
has no children, then a listener is attached. Later, when freeing the
children list, it assumed again that each node without children would
have a listener attached. This caused a crash when a node had children
before, but lost them in the meantime. Now we tried to remove a listener
where there never was one.
*many* years, so the property browser always leaked memory. I activated
this line in property_list.cxx and ... got a crash right there. So this
was the reason for it being commented out? Doing the same for now, until
I know the exact reason and can really fix it.
