NullPointerException

java.lang.NullPointerException
    at com.teamtreehouse.courses.dao.Sql2oCourseDaoTest.tearDown(Sql2oCourseDaoTest.java:28)    

package com.teamtreehouse.courses.dao;

import com.teamtreehouse.courses.model.Course;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.sql2o.Connection;
import org.sql2o.Sql2o;

import static org.junit.Assert.*;

public class Sql2oCourseDaoTest {

    private Sql2oCourseDao dao;
    private Connection conn;

    @Before
    public void setUp() throws Exception {
        String connectionString = "jdbc:h2:mem:testing;INIT=RUNSCRIPT from 'classpath:db/init.sql'";
        Sql2o sql2o = new Sql2o(connectionString);
        dao = new Sql2oCourseDao(sql2o);
        //keep connection open through entire test so that it isn't wiped out.
        conn = sql2o.open();
    }

    @After
    public void tearDown() throws Exception {
        conn.close();
    }

    @Test
    public void addingCourseSetsId() throws Exception {
        Course course = new Course("Test", "http://test.com");
        int originalCourseId = course.getId();

        dao.addCourse(course);

        //adding a course to the database should change it's id:
        assertNotEquals(originalCourseId, course.getId());
    }
}

import org.sql2o.Sql2oException;

import java.util.List;

public class Sql2oCourseDao implements CourseDao {
    private List<Course> courses;
    private final Sql2o sql2o;

    public Sql2oCourseDao(Sql2o sql2o) {
        this.sql2o = sql2o;
    }

    @Override
    public void addCourse(Course course) throws DaoException {
        //string to hold our SQL query:
        String sql = "INSERT INTO courses(name, url) VALUES (:name, :url)";
        try (Connection con = sql2o.open()) {
            //executing the SQL query and getting the id that was returned
            int id = (int) con.createQuery(sql)
                    .bind(course)
                    .executeUpdate()
                    .getKey();

            //setting the course id equal to the id returned from our database:
            course.setId(id);
        } catch (Sql2oException exc) {
            //throwing a DaoException if anything went wrong with our database connection:
            throw new DaoException(exc, "Problem adding course");
        }

    }

    @Override
    public List<Course> findAll() {
        return this.courses;
    }
}