Testing function throws UnhandledPromiseRejectionWarning. Causing test not to pass

I played with Mocha Testing. I have noticed that my function throws Warring of "UnhandledPromiseRejectionWarning" in few places. It makes the script to do not pass the check.

I could not find useful example on the internet that will teach/show a good way to fix the problem. Maybe one of you can help.

If you have any other comments to my code please feel free to share. I am here to learn :)

Function where the problem occurs.

it('/POST /logout => Logout a user by purging a session', (done) => {
        let loginInfo = {};
        loginInfo.usr = 'testuser';
        loginInfo.psw = 'mochatesting197';
        let agent = chai.request.agent(app);
        let json = {};
        json.logout = true;
        agent.post('/login')
            .send(loginInfo)
            .then((res) => {
                return agent.post('/logout')
                    .send(json)
                    .then((res) => {
                        res.should.have.status(200);
                        res.body.should.be.a('object');
                        res.body['success'].should.have.property('message').eql('YOU HAVE LOGGED OUT');
                        done();
                    }).catch(function (err) {
                        throw err;
                    });
            });
    });