add: if (!itemToDelete) { return; } 
context:     deleteOccurrence: function (aOccurrence) {        var itemToDelete = getOccurrenceOrParent(aOccurrence);        if (itemToDelete.parentItem != itemToDelete) {            var event = itemToDelete.parentItem.clone();            event.recurrenceInfo.removeOccurrenceAt(itemToDelete.recurrenceId);            doTransaction('modify', event, event.calendar, itemToDelete.parentItem, null);        } else {            doTransaction('delete', itemToDelete, itemToDelete.calendar, null, null);        }    }
meta: 9421	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/9421/02936111e056c37c612c5b52c49578d3da91e548/calendar-management.js/buggy/calendar/lightning/content/calendar-management.js