#vulnerable code 
                                @Test
    public void canUpsertWithWriteConcern() throws Exception {

        WriteConcern writeConcern = spy(WriteConcern.SAFE);

        /* when */
        WriteResult writeResult = collection.upsert("{}", "{$set:{name:'John'}}", writeConcern);

        /* then */
        People john = collection.findOne("{name:'John'}").as(People.class);
        assertThat(john.getName()).isEqualTo("John");
        assertThat(writeResult).isNotNull();
        verify(writeConcern).callGetLastError();
    } 
                            #location 11 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void canUpsertWithWriteConcern() throws Exception {

        /* when */
        WriteResult writeResult = collection.update("{}").upsert().concern(WriteConcern.SAFE).with("{$set:{name:'John'}}");

        /* then */
        People john = collection.findOne("{name:'John'}").as(People.class);
        assertThat(john.getName()).isEqualTo("John");
        assertThat(writeResult).isNotNull();
        assertThat(writeResult.getLastConcern()).isEqualTo(WriteConcern.SAFE);
    }