Merge branch 'fix-saving-calendar-events-without-recurrence' into 'master'

Fix saving CalendarEvents without recurrence

See merge request !1778

aleksis/core/models.py

@@ -1934,6 +1934,10 @@ class CalendarEvent(
 
             self.rrule_until = rule.until
         else:
+            self.rrule = None
+            self.rrule_until = None
+            self.exdatetimes = []
+            self.rdatetimes = []
             self.timezone = None
 
         super().save(*args, **kwargs)