Fix: graphknowledge Tree structure not found for treeKey: combo (#7819) (#8862)

### What problem does this PR solve?

Fixed graphknowledge Tree structure not found for treeKey.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/pages/add-knowledge/components/knowledge-graph/util.ts

   const nextNodes = nodes.map((x) => {
     return {
       ...x,
-      combo: combos.find((y) => y.data.label === findCombo(x?.communities))?.id,
+      combo:
+        combos.find((y) => y.data.label === findCombo(x?.communities))?.id ??
+        null,
     };
   });