Only listen for events on existing sites

lib/models/Site.dart

@@ -93,6 +93,7 @@ class Site {
     this.rawConfig = rawConfig;
     this.lastManagedUpdate = lastManagedUpdate;
 
+    if (id != null) {
       _updates = EventChannel('net.defined.nebula/$id');
       _updates.receiveBroadcastStream().listen((d) {
         try {
@@ -108,6 +109,7 @@ class Site {
         _change.addError(error.message ?? 'An unexpected error occurred');
       });
     }
+  }
 
   factory Site.fromJson(Map<String, dynamic> json) {
     var decoded = Site._fromJson(json);