invalidate url field for irrelevant monitor types

server/model/monitor.js

@@ -1493,6 +1493,12 @@ class Monitor extends BeanModel {
         if (this.interval < MIN_INTERVAL_SECOND) {
             throw new Error(`Interval cannot be less than ${MIN_INTERVAL_SECOND} seconds`);
         }
+
+        // Ensure URL is set to null for monitor types that don't use it,
+        // or the URL may be mistakenly used later (e.g., in notifications)
+        if (![ "http", "keyword", "json-query", "real-browser" ].includes(this.type)) {
+            this.url = null;
+        }
     }
 
     /**