getWriteableDatabase（）引发空指针异常

问题描述

尝试打开要使用Null Pointer Exception编写的SQLiteDatabase时出现崩溃。 这是我实例化SQLiteOpenHelper的方法：

private final OpenHelper mOpenHelper;

public static DataManager getInstance() {
    if (sInstance == null) {
        sInstance = new DataManager();
    }
    return sInstance;
}

private DataManager() {
    mOpenHelper = new OpenHelper(MainApp.getInstance(), MainApp.getInstance().getWalletToken());
    SQLiteDatabase.loadLibs(MainApp.getInstance());
}

MainApp是我的应用程序对象。

这是我想做的事情：

    SQLiteDatabase db = null;
    try {
        db = mOpenHelper.getWritableDatabase();

        for (Card card : cards) {
            if (card == null) {
                continue;
            }

            addCard(card, db);

            ContentValues values = new ContentValues();
            values.put(DataEntry.COLUMN_NAME_NICKNAME, card.getNickname());

            File imageFile = getImageFile(Integer.toString(card.getIndex()));
            if (ImageUtils.saveImage(card.getCardImage(), imageFile)) {
                values.put(DataEntry.COLUMN_NAME_IMAGE_FILE, imageFile.getAbsolutePath());
            }

            String whereClause = DataEntry.COLUMN_NAME_CARD_INDEX + " = "
                    + card.getIndex();
            db.update(DataEntry.TABLE_NAME, values, whereClause, null);
        }
    } finally {
        if (db != null) {
            db.close();
        }
    }        

这是崩溃输出：

03-10 16:49:10.533: E/AndroidRuntime(8771): FATAL EXCEPTION: main
03-10 16:49:10.533: E/AndroidRuntime(8771): java.lang.NullPointerException
03-10 16:49:10.533: E/AndroidRuntime(8771):     at net.sqlcipher.database.SQLiteDatabase.<init>(SQLiteDatabase.java:1957)
03-10 16:49:10.533: E/AndroidRuntime(8771):     at net.sqlcipher.database.SQLiteDatabase.openDatabase(SQLiteDatabase.java:901)
03-10 16:49:10.533: E/AndroidRuntime(8771):     at net.sqlcipher.database.SQLiteDatabase.openOrCreateDatabase(SQLiteDatabase.java:944)
03-10 16:49:10.533: E/AndroidRuntime(8771):     at net.sqlcipher.database.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:107)
03-10 16:49:10.533: E/AndroidRuntime(8771):     at com.mywebsite.DataManager$OpenHelper.getWritableDatabase(DataManager.java:282)

我用Google搜索了源代码，但该特定行不包含任何应引发异常的内容。 我究竟做错了什么？