Don't fail on non-existing version object on automatic plan update

aleksis/apps/chronos/models.py

@@ -1261,6 +1261,10 @@ class AutomaticPlan(LiveDocument):
                 )
             update = False
             for version in versions:
+                if not version.object:
+                    # Object exists no longer, so we can skip this
+                    continue
+
                 # Check if the changed object is relevant for the time period of the PDF file
                 if isinstance(version.object, Event):
                     date_start = version.object.date_start