Subscription field must return Async Iterable. Received: undefined

I use graphql nodejs with apollo-server. When i try i got this error:

I already i returns pubsub.asyncIterator in resolver. It returns error. Another project same setting works but there it is not working. I examine some stack issues here but i couldnt solve.

Server: App.js

const express = require('express');
const mongoose = require('mongoose');
const { ApolloServer } = require('apollo-server');


const { mainSchema } = require('./graphql/schemas/index');
const { rootResolver } = require('./graphql/resolvers/index');

const app = express();


const server = new ApolloServer({
typeDefs: mainSchema,
resolvers: rootResolver,
subscriptions: {
         path: `/graphql`,
         onConnect: (connectionParams, webSocket, context) => {
         console.log('Client connected');
},
onDisconnect: (webSocket, context) => {
         console.log('Client disconnected')
    },

  },
});

server.listen(3000, "192.168.1.106").then(({ url }) => {
console.log(`🚀  Server ready at ${url}`);
});

Subscription Resolver

    Subscription :{
    userNotifications : withFilter(
            ()=>pubSub.asyncIterator(NEW_NOTIFICATION_CREATE),
            (payload, variables)=>{
                console.log("payload=>", payload);
                console.log("variables=>", variables);
                return true
            }
        )}}

PubSub.js

  const { PubSub } = require('graphql-subscriptions')

  const pubSub =new PubSub()

  exports.pubSub = pubSub;

I had must used subccribe keyword for subs define. Thanks me. Subscription :{ userNotifications : withFilter( subccribe: ()=>pubSub.asyncIterator(NEW_NOTIFICATION_CREATE), (payload, variables)=>{ console.log("payload=>", payload); console.log("variables=>", variables); return true } )}}