req.body and req.file are defined only first time

when i first log req.body and req.file it is defined at first console.log but when i try to access them it is undefined and i absolutely don't know why.

i am building a small social network so when i post an image with some text the post is saved in mongo even with this error

here is the errors:

TypeError: Cannot read properties of undefined (reading 'filename')
    at exports.createPost (/Users/curlynux/Documents/groupomania-p7/backend/api/controllers/postController.js:12:26)

but after this error, req.file is defined and work well ... why after ? after the error:

{
  fieldname: 'image',
  originalname: '73423630_448993115731641_5411606821110292030_n.jpg',
  encoding: '7bit',
  mimetype: 'image/jpeg',
  destination: 'images',
  filename: '73423630_448993115731641_5411606821110292030_n.jpg1663242018229.jpeg',
  path: 'images/73423630_448993115731641_5411606821110292030_n.jpg1663242018229.jpeg',
  size: 41437
}
73423630_448993115731641_5411606821110292030_n.jpg1663242018229.jpeg
POST /post 201 74.811 ms - 26

req.body is defined firstly:

{
  login: 'login',
  imageUrl: 'string',
  post_text: 'test',
  like: '0',
  disLike: '0'
}

but undefined after ?: [Object: null prototype] {}

my postController.js:

const express = require("express");
const app = express();
const Post = require("../models/postModel");

app.use(express.json())
app.use(express.urlencoded({extended: false}))

exports.createPost = (req, res) => 
{
    console.log(req.body);
    console.log(req.file);
    console.log(req.file.filename);
    const post = new Post({post: {
        login: req.body.login,
        imageUrl: `${req.protocol}://${req.get("host")}/images/${req.file.filename}`,
        post_text: req.body.post_text,
        like: req.body.like,
        disLike: req.body.disLike
    }})
    post.save().then(() => res.status(201).json({message: "post created"}))
    
}

chrome error: POST http://localhost:8080/post 500 (Internal Server Error) home.jsx:69

the corresponding code for the chrome error:

try
        {
            fetch("http://localhost:8080/post", 
            {
                method: "POST",
                mode: "cors",
                headers: postHeader,
                body: JSON.stringify(postData)
            }).then(response => console.log(response))
            .then(data => console.log(data))
        } catch (error)
        {console.error(error)}