output: #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);
      }
input: #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