Fix prometheus null issues

server/model/monitor.js

@@ -755,7 +755,7 @@ class Monitor extends BeanModel {
             await R.store(bean);
 
             log.debug("monitor", `[${this.name}] prometheus.update`);
-            this.prometheus.update(bean, tlsInfo);
+            this.prometheus?.update(bean, tlsInfo);
 
             previousBeat = bean;
 
@@ -840,7 +840,7 @@ class Monitor extends BeanModel {
         clearTimeout(this.heartbeatInterval);
         this.isStop = true;
 
-        this.prometheus.remove();
+        this.prometheus?.remove();
     }
 
     /**