@Test
public void tryToTestNullJsonObject() {

    final Function<String, String> jsonKeyNameReviser = Mockito.mock(Function.class);
    final Function<JsonObject, BsonDocument> underTest =
            JsonValueToDbEntityMapper.forJsonObject(jsonKeyNameReviser);
    assertThatExceptionOfType(NullPointerException.class)
            .isThrownBy(() -> underTest.apply(null))
            .withMessage("The %s must not be null!", "JSON object to be mapped")
            .withNoCause();
}