diff --git a/src/ui/src/components/visualizer/app_service.ts b/src/ui/src/components/visualizer/app_service.ts
index a57ca882..6d44f55f 100644
--- a/src/ui/src/components/visualizer/app_service.ts
+++ b/src/ui/src/components/visualizer/app_service.ts
@@ -172,7 +172,10 @@ export class AppService {
               }
               graph.subGraphIds.push(...node.subgraphIds);
               for (const subgraphId of node.subgraphIds) {
-                graphById[subgraphId].parentGraphId = graph.id;
+                const subgraph = graphById[subgraphId];
+                if (subgraph) {
+                  subgraph.parentGraphId = graph.id;
+                }
               }
             }
           }
@@ -190,7 +193,9 @@ export class AppService {
           if (root == null) {
             graphs = rootGraphs;
           } else {
-            graphs = (root.subGraphIds || []).map((id) => graphById[id]);
+            graphs = (root.subGraphIds || [])
+              .map((id) => graphById[id])
+              .filter((graphs) => graphs != null);
           }
           // Sort by node count.
           graphs.sort((g1, g2) => g2.nodes.length - g1.nodes.length);
diff --git a/src/ui/src/components/visualizer/webgl_renderer.ts b/src/ui/src/components/visualizer/webgl_renderer.ts
index ec1b4f5f..3e0f4560 100644
--- a/src/ui/src/components/visualizer/webgl_renderer.ts
+++ b/src/ui/src/components/visualizer/webgl_renderer.ts
@@ -2820,6 +2820,14 @@ export class WebglRenderer implements OnInit, OnDestroy {
   }
 
   private async openSubgraph(subgraphId: string) {
+    const graph = this.appService.getGraphById(subgraphId);
+    if (!graph) {
+      const msg = `No graph found for subgraph id: "${subgraphId}"`;
+      console.warn(msg);
+      this.snackBar.open(msg, 'Dismiss');
+      return;
+    }
+
     // Add breadcrumb.
     this.appService.addSubgraphBreadcrumbItem(
       this.paneId,
@@ -2829,14 +2837,11 @@ export class WebglRenderer implements OnInit, OnDestroy {
     );
 
     // Open the subgraph in current pane.
-    const graph = this.appService.getGraphById(subgraphId);
-    if (graph) {
-      this.appService.selectNode(this.paneId, undefined);
-      this.appService.setFlattenLayersInCurrentPane(false);
-      this.appService.curInitialUiState.set(undefined);
-      this.appService.curToLocateNodeInfo.set(undefined);
-      this.appService.selectGraphInCurrentPane(graph);
-    }
+    this.appService.selectNode(this.paneId, undefined);
+    this.appService.setFlattenLayersInCurrentPane(false);
+    this.appService.curInitialUiState.set(undefined);
+    this.appService.curToLocateNodeInfo.set(undefined);
+    this.appService.selectGraphInCurrentPane(graph);
   }
 
   private getGroupNodeLabelSeparatorId(