读取/同步记录时，Android Sync Null异常

Question

我想将我的SQL Server数据库与android SQLite同步。

我已按照链接http://programmerguru.com/android-tutorial/how-to-sync-remote-mysql-db-to-sqlite-on-android/

上提供的教程进行操作

不同之处在于我使用Microsoft Visual Studio Web Service代替PHP。

我已复制代码并根据我的要求进行了更改。现在，每当我运行移动应用程序时，我都会收到以下错误：

  

无法启动活动ComponentInfo {com.murcyleenpeerzada.mp/com.murcyleenpeerzada.mp.writings}：java.lang.NullPointerException：尝试调用虚方法'java.util.ArrayList com.murcyleenpeerzada.mp.DBHandler.getAllWriters （）'在空对象引用

上

我检查了它有记录的表格。该表不是空白。

我在DBHandler中的代码是：

    public ArrayList<HashMap<String, String>> getAllWriters() {
    ArrayList<HashMap<String, String>> usersList;
    usersList = new ArrayList<HashMap<String, String>>();
    String selectQuery = "SELECT * FROM " + MP_TABLE;
    SQLiteDatabase database = this.getWritableDatabase();
    Cursor cursor = database.rawQuery(selectQuery, null);
    if (cursor.moveToFirst()) {
        do {
            HashMap<String, String> map = new HashMap<String, String>();
            map.put("writeId", cursor.getString(0));
            map.put("title", cursor.getString(1));
            usersList.add(map);
        } while (cursor.moveToNext());
    }
    database.close();
    return usersList;
}

我的活动代码是：

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.writings);

    // Get User records from SQLite DB
    ArrayList<HashMap<String, String>> userList = dbHandler.getAllWriters();
    // If users exists in SQLite DB
    if (userList.size() != 0) {
        // Set the User Array list in ListView
        ListAdapter adapter = new SimpleAdapter(getApplicationContext(), userList, R.layout.writings_controls, new String[] {
                "userId", "userName" }, new int[] { R.id.writings_Title, R.id.writings_Des});
        ListView myList = (ListView) findViewById(android.R.id.list);
        myList.setAdapter(adapter);
    }