JUnit-java。lang.NullPointerException:无法调用“…”因为“this.modelMapper”为空

我学Java已经四个月了,所以请原谅我的基本错误。

我正在尝试从我的服务层对一个方法进行单元测试:

    @Override
    @Transactional
    public List<StudentDto> getStudentList() {
        List<Student> tempList = studentDAO.getStudentList();

        List<StudentDto> tempListStudentDto = new ArrayList<>();

        for (Student theStudent: tempList) {
            tempListStudentDto.add(convertToDto(theStudent));
        }

        return tempListStudentDto;
    }

通过这个@Test:

    // JUnit test for getStudentList()
    @Test
    @DisplayName("getStudentList()")
    public void givenStudentList_whenGettingList_thenReturnList() {
        // given -precondition
        BDDMockito.given(studentDao.getStudentList())
                .willReturn(List.of(newStudentOne, newStudentTwo));

        // when - behaviour that we are going to test

        List<StudentDto> studentList = studentService.getStudentList();

        // then - verify the output
        assertAll(
                () -> org.assertj.core.api.Assertions.assertThat(studentList).isNotNull(),
                () -> org.assertj.core.api.Assertions.assertThat(studentList).size().isEqualTo(2)
        );

我经常会犯这样的错误: JAVAlang.NullPointerException:无法调用“org.modelmapper.modelmapper.getConfiguration()”,因为“this.modelmapper”为空

你能在这里帮助我吗,或者告诉我如何以可测试的方式将学生类转换为DTO?

我在谷歌上搜索了这个问题,尝试了所有的建议,但都没有成功。