I'm trying to code the authentification part of my react app using jwt. I kept getting this error in my App.js file: 'Uncaught (in promise) TypeError: Cannot read properties of null (reading 'error')', which led me to find out that when a certain route is accessed, which should return some data about the user, the response body is actually empty. These are some snippets of my code, where I think the problem might be. Any help would be much appreciated, thanks in advance! Also, please let me know if I should provide other snippets of code

This is from my App.js file

const [authState, setAuthState] = useState({
    username: "", 
    id: 0, 
    status: false,
  });

  useEffect(() => {
    axios
      .get("http://localhost:3001/users/auth", {
        headers: {
          accessToken: localStorage.getItem("accessToken"),
        },
      })
      .then((response) => {
        if (response.data.error) { //this is where the error appears
          setAuthState({ ...authState, status: false });
        } else {
          setAuthState({
            username: response.data.username, //if I comment the line where the error appears, I get the same error here and on the line below, but with 'username' and 'id' instead of 'error'
            id: response.data.id,
            status: true,
          });
        }
      });
  }, []);

This is where res.json doesn't return anything

router.get('/auth', validateToken, (req, res) => {
    res.json(req.user);
});

This is the validateToken middleware

const { verify } = require('jsonwebtoken');

const validateToken = (req, res, next) => {
    const accessToken = req.header("accessToken");

    if (!accessToken) return res.json({ error: "User not logged in!" })

    try {
        const validToken = verify(accessToken, "importantsecret");
        req.user = validToken; //we can access the username and id
        if (validToken) {
            return next();
        }
    } catch (err) {
        return res.json({ error: err });
    }
};

module.exports = { validateToken };