fix: add runtime checks for graph accessors in multiple classes

geaflow-ai/src/main/java/org/apache/geaflow/ai/GeaFlowMemoryServer.java

@@ -160,8 +160,12 @@ public String addEntity(@Param("graphName") String graphName,
                 memoryMutableGraph.addEdge(((GraphEdge) entity).getEdge());
             }
         }
+        GraphMemoryServer insertServer = CACHE.getServerByName(graphName);
+        if (insertServer == null || insertServer.getGraphAccessors().isEmpty()) {
+            throw new RuntimeException("Server or graph accessor not available for graph: " + graphName);
+        }
         CACHE.getConsolidateServer().executeConsolidateTask(
-            CACHE.getServerByName(graphName).getGraphAccessors().get(0), memoryMutableGraph);
+            insertServer.getGraphAccessors().get(0), memoryMutableGraph);
         return "Success to add entities, num: " + graphEntities.size();
     }
 
@@ -213,6 +217,9 @@ public String execQuery(@Param("sessionId") String sessionId,
         VectorSearch search = new VectorSearch(null, sessionId);
         search.addVector(new KeywordVector(query));
         server.search(search);
+        if (server.getGraphAccessors().isEmpty()) {
+            throw new RuntimeException("No graph accessor available for session: " + sessionId);
+        }
         Context context = server.verbalize(sessionId,
             new SubgraphSemanticPromptFunction(server.getGraphAccessors().get(0)));
         return context.toString();

geaflow-ai/src/main/java/org/apache/geaflow/ai/GraphMemoryServer.java

@@ -81,6 +81,9 @@ public String search(VectorSearch search) {
             sessionManagement.createSession(sessionId);
         }
 
+        if (graphAccessors.isEmpty()) {
+            throw new RuntimeException("No graph accessor available");
+        }
         for (IndexStore indexStore : indexStores) {
             if (indexStore instanceof EntityAttributeIndexStore) {
                 SessionOperator searchOperator = new SessionOperator(graphAccessors.get(0), indexStore);

geaflow-ai/src/main/java/org/apache/geaflow/ai/service/ServerMemoryCache.java

@@ -38,11 +38,17 @@ public void putGraph(Graph g) {
     }
 
     public void putServer(GraphMemoryServer server) {
+        if (server.getGraphAccessors().isEmpty()) {
+            throw new RuntimeException("Cannot register server without graph accessor");
+        }
         name2Server.put(server.getGraphAccessors().get(0)
             .getGraphSchema().getName(), server);
     }
 
     public void putSession(GraphMemoryServer server, String sessionId) {
+        if (server.getGraphAccessors().isEmpty()) {
+            throw new RuntimeException("Cannot register session without graph accessor");
+        }
         session2GraphName.put(sessionId,
             server.getGraphAccessors().get(0).getGraphSchema().getName());
     }