Check If existing doesn't work after sort

Question

My mongoDB findOne which was working before because it was responsible for checking if an email exist in my database, but as soon i added the .sort to it, it throws an error UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'email' of null at User.findOne.sort.then.user But if i remove the sort, it will work again including the email do exist validation.

Here is the code

router.post('/register', (req, res) => {
  const { errors, isValid } = validateRegisterInput(req.body);

  // Check Validation 
  if (!isValid) {
    return res.status(400).json(errors);
  }

  User.findOne({ email : req.body.email }).sort({unitNo : -1}).then(user => {
    if (user.email === req.body.email) {
    errors.email = 'Email already exists';
     return res.status(400).json(errors);
    } else {
      // create my secret token for email verification
        const secretToken = randomstring.generate();
        const defaultNo = user[0].unitNo;
        const unitNo = String(Number(defaultNo)+1).padStart(4, '0');
        const newUser = new User(
      {
        email: req.body.email,
        password: req.body.password,
        phone : req.body.phone,
        fullname: req.body.fullname,
        secretToken,
        unitNo,
        });
      const DOMAIN = "******.mailgun.org";
      const mg = mailgun({apiKey: "*******", domain: DOMAIN});
      const data = {
                    from: 'no-reply@yourwebapplication.com', 
                    to: newUser.email,
                    subject: 'Account Verification Token', 
                    text: 'Hello,\n\n' + 'Please verify your account by clicking the link: \nhttp:\/\/' + req.headers.host + '/\api/users/verify\/' + newUser.secretToken + '.\n' 
                   };
      mg.messages().send(data, function (error, body) {
    console.log(body);
});
     bcrypt.genSalt(10, (err, salt) => {
        bcrypt.hash(newUser.password, salt, (err, hash) => {
          if (err) throw err;
          newUser.password = hash;
            newUser 
             .save()
              .then(user => res.json(user))
              .catch(err => console.log(err));
        });
      });
    }
  });
});