Cannot read property 'getIdToken' of null after implementing session cookies in firebase

I implemented cookie sessions on my firebase project using this link.
I'm trying to get the idToken in the client side to send it to server side.
Here's the code I'm using on client side:

$(function(){
    $.ajax({
        url: "/checkSession",
        type: 'post',
        success: function(data) {
            if(data.status === "ok" && data.user){
                const user = data.user;
                const emailVerified = user.email_verified;
                const uid = user.uid;

                if (emailVerified){
                    firebase.database().ref().child("users").child(uid).child("username").once("value", snapshot => {
                        const username = snapshot.val();
                        firebase.auth().currentUser.getIdToken(true).then(idToken => {
                        // Doesn't work...
                        })
                    }).catch(err => {
                        console.log(err)
                    });
                }
            }
        }
    }); 
});

And here's the server that checks for the session:

app.post("/checkSession", (req, res, next) => {
    const sessionCookie = req.cookies.session || '';
    admin
        .auth()
        .verifySessionCookie(sessionCookie, true)
        .then(user => {
            res.json({
                status: "ok",
                user
            })
        })
        .catch((error) => {
            res.json({
                status: "error",
            })
        });
})

Everything works except this function: firebase.auth().currentUser.getIdToken(true).then(idToken => {...}) and it's throwing Cannot read property 'getIdToken' of null error.

UPDATE: Here's the sign-in method I'm using. I'm passing false to httpOnly and secure since I'm testing it locally.

app.post("/SignInUser", (req, res, next) => {
    const idToken = req.body.idToken.toString();
    const expiresIn = 60 * 60 * 24 * 10 * 1000; // 10 days

    admin
        .auth()
        .verifyIdToken(idToken)
        .then(function (decodedClaims) {
            if (new Date().getTime() / 1000 - decodedClaims.auth_time < 5 * 60) {
                return admin.auth().createSessionCookie(idToken, { expiresIn });
            }
        })
        .then(function (sessionCookie) {
            const options = { maxAge: expiresIn, httpOnly: false, secure: false};
            res.cookie("session", sessionCookie, options);
            return res.json({
                status: "ok"
            })
        })
        .catch(function (error) {
            return res.json({
                status: "error",
                message: "Unauthorized request!"
            })
        });
})

UPDATE 2: I use the idToken to verify that the request coming from is actually from the user and not from an external source, here's an example code:

function getUserSnapshotOrVerifyUserId(username, idToken, cb) {
    if (username == null || username.length == 0 || idToken == null || idToken.length == 0)
        return cb({
            status: "error",
            errorMessage: "Missing params."
        }, null);

    admin.auth().verifyIdToken(idToken).then(decodedToken => {
        let uid = decodedToken.uid;

        admin.database().ref().child("users").orderByChild("username").equalTo(username).once('value', snapshot => {
            if (!snapshot.exists())
                return cb({
                    status: "error",
                    message: "invalid-profile"
                });

            snapshot.forEach(child => {
                const id = child.val().id;
                if (id !== uid)
                    return cb({
                        status: "error",
                        message: "Invalid ID"
                    });

                admin.database().ref("users/" + id).once("value", snapshot => {
                    if (!snapshot.exists())
                        return cb({
                            status: "error",
                            errorMessage: "user not found."
                        });

                    return cb(null, id, snapshot);
                });
            });
        });
    }).catch(err => cb({
        status: "error",
        message: err
    }));
}

app.post("/getUserProfile", (req, res, next) => {
    const username = req.body.username || req.query.username;
    const idToken = req.body.idToken;

    getUserSnapshotOrVerifyUserId(username, idToken, (err, id, snapshot) => {
        if (err) return res.json(err);

        let userdata = {
            username: snapshot.val().username,
            name: snapshot.val().name,
        }

        res.json({
            status: "ok",
            userdata
        })
    })
})

from Cannot read property 'getIdToken' of null after implementing session cookies in firebase