SOLVED : TypeError: Cannot return null for non-nullable field Subscription: Next.js frontend + FastApi backend about apollo-client HOT 3 CLOSED

"use client";
import {ApolloLink, HttpLink, split} from "@apollo/client";
import {
    ApolloNextAppProvider,
    NextSSRInMemoryCache,
    NextSSRApolloClient,
    SSRMultipartLink,
} from "@apollo/experimental-nextjs-app-support/ssr";
import React from "react";
import {GraphQLWsLink} from "@apollo/client/link/subscriptions";
import {createClient} from "graphql-ws";
import {getMainDefinition} from "@apollo/client/utilities";

function makeClient() {
    const httpLink = new HttpLink({
        uri: "http://localhost:8000/graphql",
    });
    const wsLink = new GraphQLWsLink(createClient({
        url: 'ws://localhost:8000/graphql',
        connectionParams: {
            authToken: "Bearer I_AM_A_VALID_AUTH_TOKEN",
        },
    }));

    const splitLink = split(
        ({ query }) => {
            const definition = getMainDefinition(query);
            return (
                definition.kind === 'OperationDefinition' &&
                definition.operation === 'subscription'
            );
        },
        wsLink,
        httpLink,
    );

    return new NextSSRApolloClient({
        cache: new NextSSRInMemoryCache(),
        link:
            typeof window === "undefined"
                ? ApolloLink.from([
                    new SSRMultipartLink({
                        stripDefer: true,
                    }),
                    splitLink,
                ])
                : splitLink,
    });
}

export function ApolloWrapper({ children }) {
    return (
        <ApolloNextAppProvider makeClient={makeClient}>
            {children}
        </ApolloNextAppProvider>
    );
}