frontend-app-api: fix AppNode serialization

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/silent-chairs-smoke.md

@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-app-api': patch
+---
+
+Fix for app node output IDs not being serialized correctly.

packages/frontend-app-api/src/tree/resolveAppTree.ts

@@ -65,7 +65,7 @@ class SerializableAppNode implements AppNode {
     const dataRefs = this.instance && [...this.instance.getDataRefs()];
     const out =
       dataRefs && dataRefs.length > 0
-        ? ` out=[${[...dataRefs.keys()].join(', ')}]`
+        ? ` out=[${[...dataRefs].map(r => r.id).join(', ')}]`
         : '';
 
     if (this.edges.attachments.size === 0) {