ผมเอาบทความ Android ListView and Checkbox มาปรับใช้กับงานของผม แต่มันเกิด error NullPointerException อยากขอคำแนะนำหน่อยว่าผมทำอะไรตรงไหนพลาดไป

public class PromotionTypeList extends Activity {

    private static final String TAG_PROMOTION_Type = "promotion_type";
    private static final String TAG_PROMOTION_Type_ID = "proTypeID";
    private static final String TAG_PROMOTION_Type_NAME = "proTypeName";

    JSONArray PromoTypeArray = null;

    public String getJson= null;

    ListView listView;
    SimpleAdapter adapter;

    Button btnGetType;

    ArrayList<HashMap<String, String>> promotionTypeList = new ArrayList<HashMap<String, String>>();

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

        ActionBar actionBar = getActionBar();
        actionBar.setTitle("Promotion Type");

        listView = (ListView) findViewById(R.id.listView4);

        getJson = getIntent().getStringExtra("PromotionType");
        Log.d("GetJson", getJson);

        JSONObject jsonObject = null;

        try {
            PromoTypeArray = jsonObject.getJSONArray(TAG_PROMOTION_Type);

            if (PromoTypeArray.length() == 0) {
                AlertDialog.Builder dialog = new AlertDialog.Builder(this);
                dialog.setTitle("Caution!");
                dialog.setIcon(R.drawable.ic_action_error);
                dialog.setCancelable(false);
                dialog.setMessage("No Promotion Type Data.");
                dialog.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        Intent i = new Intent(getApplicationContext(), Find_Promotion.class);
                        startActivity(i);
                        finish();
                    }
                });
                dialog.show();
            } else {
                Log.d("SIZE ARRAY", String.valueOf(PromoTypeArray.length()));
                for (int i = 0; i < PromoTypeArray.length(); i++) {
                    JSONObject object = PromoTypeArray.getJSONObject(i);

                    String proType_id = object.getString(TAG_PROMOTION_Type_ID);
                    String proType_name = object.getString(TAG_PROMOTION_Type_NAME);

                    Log.d("proType_id", proType_id);
                    Log.d("proType_name", proType_name);

                    HashMap<String, String> proType = new HashMap<String, String>();

                    proType.put(TAG_PROMOTION_Type_ID, proType_id);
                    proType.put(TAG_PROMOTION_Type_NAME, proType_name);

                    promotionTypeList.add(proType);
                }

            }
        } catch (JSONException e) {
            Log.d("JSON PARSER", "Parsing Error");
        }

        listView.setAdapter(new CountryAdapter(this));

        btnGetType = (Button) findViewById(R.id.protypeToFindPro);
        btnGetType.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                int countArray = listView.getAdapter().getCount();
                Log.d("count", String.valueOf(countArray));

                for (int j = 0; j < countArray; j++) {
                    RelativeLayout itemLayout = (RelativeLayout)listView.getChildAt(j);
                    
                    if(checkbox.isChecked())
                    {
                        Log.d("Item "+String.valueOf(j), checkbox.getTag().toString());

                        Toast.makeText(PromotionTypeList.this,checkbox.getTag().toString() ,Toast.LENGTH_LONG).show();
                    }
                }
            }
        });
    }

    public class CountryAdapter extends BaseAdapter {
        private Context context;

        public CountryAdapter(Context c) {
            //super( c, R.layout.activity_column, R.id.rowTextView, );
            // TODO Auto-generated method stub
            context = c;
        }

        public int getCount() {
            // TODO Auto-generated method stub
            return promotionTypeList.size();
        }

        public Object getItem(int position) {
            // TODO Auto-generated method stub
            return position;
        }

        public long getItemId(int position) {
            // TODO Auto-generated method stub
            return position;
        }

        public View getView(final int position, View convertView, ViewGroup parent) {
            // TODO Auto-generated method stub

            LayoutInflater inflater = (LayoutInflater) context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

            if (convertView == null) {
                convertView = inflater.inflate(R.layout.promotion_type_item, null);

            }

            // Type ID
            TextView txtID = (TextView) convertView.findViewById(R.id.promotionTypeId);
            txtID.setText(promotionTypeList.get(position).get("proTypeID") + ".");

            // Type Name
            TextView txtCode = (TextView) convertView.findViewById(R.id.promotionTypeName);
            txtCode.setText(promotionTypeList.get(position).get("proTypeName"));

            // Type Check
            CheckBox Chk = (CheckBox) convertView.findViewById(R.id.promotionTypeChk);
            Chk.setTag(promotionTypeList.get(position).get("proTypeID"));

            return convertView;

        }
    }

    public void onBackPressed(){
        Intent intent = new Intent(getApplicationContext(), Find_Promotion.class);
        startActivity(intent);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#ffffffff">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="   OK   "
        android:id="@+id/protypeToFindPro"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true" />

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/listView4"
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true"
        android:layout_marginBottom="50dp"
        android:layout_marginTop="25dp"
        android:background="#ffffffff" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="You can select more than one"
        android:id="@+id/textView12"
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true"
        android:textSize="20dp"
        android:layout_marginLeft="5dp"/>
</RelativeLayout>