尝试访问 Android 中的 SQLite 数据库时，如何修复 SQLiteOpenHelper.getDatabaseLocked 错误？

我正在尝试从 Sqlite 数据库检索一些数据。但是当我尝试使用 getTaskSent 方法检索数据时，出现以下错误。谁能建议我解决以下错误？

  E/AndroidRuntime(23159):  at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)

这是我的数据库代码：

public class Databasehandler extends SQLiteOpenHelper{
private static final String LOGCAT = null;

public Databasehandler(Context applicationcontext) {
    super(applicationcontext,"androidsqlite.db", null, 4);
    // TODO Auto-generated constructor stub
    Log.d(LOGCAT,"Database Created");

}
@Override
public void onCreate(SQLiteDatabase db) {
    // TODO Auto-generated method stub
    String query,query1,query2; 
    query = "CREATE TABLE userlogin ( phoneId INTEGER PRIMARY KEY,username TEXT,userid INTEGER,contacts TEXT UNIQUE)";
 query1 = "CREATE TABLE task ( TaskId INTEGER PRIMARY KEY,heading TEXT,desc TEXT,d1 TEXT,d2 TEXT,t1 TEXT,t2 TEXT,receiver TEXT,sender TEXT)";
 query2 = "CREATE TABLE me ( fbId INTEGER PRIMARY KEY,fbname TEXT,fbuserid INTEGER,fbpic BLOB,ph TEXT,email TEXT)"; 
 db.execSQL(query1);
    db.execSQL(query);
    db.execSQL(query2);

}
 public void inserttask(HashMap<String, String> queryValues) {
    SQLiteDatabase database = this.getWritableDatabase();
    ContentValues values2 = new ContentValues();
    values2.put("heading", queryValues.get("heading"));
    values2.put("desc", queryValues.get("desc"));
    values2.put("d1", queryValues.get("d1"));
    values2.put("d2", queryValues.get("d2"));
    values2.put("t1", queryValues.get("t1"));
    values2.put("t2", queryValues.get("t2"));
    values2.put("sender", queryValues.get("sender"));
    values2.put("receiver", queryValues.get("receiver"));

    database.insert("task", null, values2);
    database.close();
}
  public ArrayList<HashMap<String, String>> getTaskSent() {
    ArrayList<HashMap<String, String>> wordList;
    wordList = new ArrayList<HashMap<String, String>>();
    String selectQuery = "SELECT * FROM task";

    SQLiteDatabase database = this.getReadableDatabase();
    Cursor cursor = database.rawQuery(selectQuery, null);
    if (cursor.moveToFirst()) {
        do {
            HashMap<String, String> map = new HashMap<String, String>();
            //map.put("userid", cursor.getString(1));
            map.put("heading", cursor.getString(1));
            map.put("desc", cursor.getString(2));
            map.put("d1", cursor.getString(3));
            map.put("d2", cursor.getString(4));
            map.put("t1", cursor.getString(5));
            map.put("t2", cursor.getString(6)); 
            map.put("receiver", cursor.getString(7));                                                                                                           

            wordList.add(map);
        } while (cursor.moveToNext());
    }cursor.close();
    database.close();


    return wordList;
}

The Context你传递给你的Databasehandler构造函数是null.