如何在android中解析JSONArray NullPointerException

我想解析另一个活动传递的jsonObject

intent.putextra("result", json.toString());

它用名称，分支和会话正确显示所有内容。但它在获取数组时显示NullPointerException。在这儿

            mSubList.add(map);

该数组有6行。这是我的代码。请告诉我确切的地方，我错了。

JSONObject json;
ListView sub_list;

private JSONArray mComments = null;
private ArrayList<HashMap<String, String>> mSubList;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_result_page);

    name = (TextView)findViewById(R.id.name_roll);
    branch= (TextView)findViewById(R.id.branchname);
    session = (TextView)findViewById(R.id.Session);

    sub_list = (ListView)findViewById(R.id.sub_list);



    try {
         json = new JSONObject(getIntent().getStringExtra("result"));

        name.setText(json.getString("REGNO")+" - "+json.getString("STUDENTNAME"));
        branch.setText(json.getString("BRANCHNAME"));
        session.setText(json.getString("SESSIONNAME"));

        mComments = json.getJSONArray("DATA");

        // looping through all subjects according to the json object returned
        for (int i = 0; i < mComments.length(); i++) {
            JSONObject c = mComments.getJSONObject(i);

            // gets the content of each tag
            String code = c.getString("CCODE");
            String subject = c.getString("COURSENAME");
            String passfail = c.getString("PASSFAIL");

            // creating new HashMap
            HashMap<String, String> map = new HashMap<String, String>();

            map.put("CCODE", code);
            map.put("COURSENAME", subject);
            map.put("PASSFAIL", passfail);
            // adding HashList to ArrayList
            mSubList.add(map);
        }

                ListAdapter adapter = new SimpleAdapter(this, mSubList,
                R.layout.singlesubject, new String[] { "CCODE", "COURSENAME",
                "PASSFAIL" }, new int[] { R.id.sub_id, R.id.sub_name,R.id.sub_res });

        sub_list.setAdapter(adapter);
    }catch(Exception e){
        e.printStackTrace();
    }
  }
}

如果我想更改每个listitem背景（R.layout.singlesubject），如果“ PASSFAIl” = P，则为绿色，否则为红色。那我该怎么做？