The DiveEventItem had an internal copy of the event. It passed
that copy to the undo-machinery, which of course didn't work.
Simply keep a pointer to the event. All changes to a dive no
pass via the undo-machinery, which causes a reload of the profile,
so this should be safe.
Reported-by: Willem Ferguson <willemferguson@zoology.up.ac.za>
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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