Throw error when use with next@13.4.3

[joshuaavalon]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.17.0

Plugin version

9.2.0

Node.js version

20.1.0

Operating system

Windows

Operating system version (i.e. 20.04, 11.3, 10)

10

Description

Error is thrown when next is updated to 13.4.3

- error uncaughtException: TypeError: Cannot read properties of undefined (reading 'nextConfig')
    at C:\Users\user\project\git\test1\node_modules\@fastify\nextjs\index.js:31:37

Steps to Reproduce

// index.js

import createFastify from "fastify";
import nextJs from "@fastify/nextjs";

const fastify = createFastify();
const nextPlugin = nextJs.default;

await fastify.register(nextPlugin);
fastify.after(() => {
  fastify.next("/hello");
});

fastify.listen({ port: 8001 }, (err) => {
  if (err) throw err;
  console.log("Server listening on http://localhost:3000");
});

{
  "name": "test1",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "type": "module",
  "scripts": {
    "start": "node index.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@fastify/nextjs": "^9.2.0",
    "fastify": "^4.17.0",
    "next": "^13.4.3"
  }
}

Expected Behavior

No error should be thrown.

[joshuaavalon]

See to related to vercel/next.js#50315