TypeError: Cannot read properties of null (reading '_id') – how to solve this error? – Reactjs

I am getting the above error.

Model.js

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

var CourseSchema = new Schema({

    courseCode: {
        type: String,
        required: true,
        unique: true,
    },
    courseName: {
        type: String,
        required: 'Course Name cannot be blank',
    },
    section: {
        type: String,
        required: 'Section cannot be blank',
    },
    semester: {
        type: String,
        required: 'Semester cannot be blank',
    },
    creator: {
        type: Schema.ObjectId,
        ref: 'Student'
    },
    created: {
        type: Date,
        default: Date.now
    }
});

mongoose.model('Course', CourseSchema);

Controller.js

const mongoose = require('mongoose');
const Course = mongoose.model('Course');
const Student = require('mongoose').model('Student');

function getErrorMessage(err) {
    if (err.errors) {
        for (let errName in err.errors) {
            if (err.errors[errName].message) return err.errors[errName].
                message;
        }
    } else {
        return 'Unknown server error';
    }
};

exports.create = function (req, res) {
    const course = new Course();
    course.courseCode = req.body.courseCode;
    course.courseName = req.body.courseName;
    course.section = req.body.section;
    course.semester = req.body.semester;
   
    console.log(req.body)
    //
    //
    Student.findOne({studentNumber: req.body.studentNumber}, (err, student) => {

        if (err) { return getErrorMessage(err); }
        if (!student) {
            return res.status(400).send({
                message: 'No student found with the given student number'
            });
        }
        //
        console.log('student._id',student._id);
        req.id = student._id;
        console.log('student._id',req.id);

    
    }).then( function () 
    {
        course.creator = req.id
        console.log('req.student._id',req.id);

        course.save((err) => {
            if (err) {
                console.log('error', getErrorMessage(err))

                return res.status(400).send({
                    message: getErrorMessage(err)
                });
            } else {
                res.status(200).json(course);
            }
        });
    
    });
};

//
exports.list = function (req, res) {
    Course.find().sort('-created').populate('creator', 'firstName lastName fullName').exec((err, courses) => {
if (err) {
        return res.status(400).send({
            message: getErrorMessage(err)
        });
    } else {
        res.status(200).json(courses);
    }
});
};

exports.courseByID = function (req, res, next, id) {
    Course.findById(id).populate('creator', 'firstName lastName fullName').exec((err, course) => {if (err) return next(err);
    if (!course) return next(new Error('Failed to load course '
            + id));
        req.course = course;
        console.log('in courseById:', req.course)
        next();
    });
};

exports.read = function (req, res) {
    res.status(200).json(req.course);
};

exports.update = function (req, res) {
    console.log('in update:', req.course)
    const course = req.course;
    course.courseCode = req.body.courseCode;
    course.courseName = req.body.courseName;
    course.section = req.body.section;
    course.semester = req.body.semester;

    course.save((err) => {
        if (err) {
            return res.status(400).send({
                message: getErrorMessage(err)
            });
        } else {
            res.status(200).json(course);
        }
    });
};

exports.delete = function (req, res) {
    const course = req.course;
    course.remove((err) => {
        if (err) {
            return res.status(400).send({
                message: getErrorMessage(err)
            });
        } else {
            res.status(200).json(course);
        }
    });
};

//The hasAuthorization() middleware uses the req.course and req.student objects
//to verify that the current student is the creator of the current course
exports.hasAuthorization = function (req, res, next) {
    console.log('in hasAuthorization - creator: ',req.course.creator)
    console.log('in hasAuthorization - student: ',req.id)
    //console.log('in hasAuthorization - student: ',req.student._id)


    if (req.course.creator.id !== req.id) {
        return res.status(403).send({
            message: 'Student is not authorized'
        });
    }
    next();
};

I am able to login and logout. But when I add course it says that id is null. When i print it on console, it looks fine.
What is the problem? i am using ref to allow a student document to make a reference to corresponding course document.