bugfix: do not make event if target is nil

src/server/engine/worker.go

@@ -371,6 +371,9 @@ func (r *RuleEval) MakeNewEvent(from string, now int64, clusterName string, vect
 					logger.Debugf("event_enable_in_bg: rule_eval:%d", r.rule.Id)
 					continue
 				}
+			} else {
+				// target 已经不存在了，可能是被删除了
+				continue
 			}
 		}