Android：无法执行此操作，因为连接池已关闭

我正在阅读StackOverflow关于这个问题，我仍然没有找到解决方案。我注意到有时，我的应用程序会抛出此错误：

   java.lang.IllegalStateException: Cannot perform this operation because the connection pool has been closed.
        at android.database.sqlite.SQLiteConnectionPool.throwIfClosedLocked(SQLiteConnectionPool.java:962)
        at android.database.sqlite.SQLiteConnectionPool.waitForConnection(SQLiteConnectionPool.java:599)
        at android.database.sqlite.SQLiteConnectionPool.acquireConnection(SQLiteConnectionPool.java:348)
        at android.database.sqlite.SQLiteSession.acquireConnection(SQLiteSession.java:894)
        ...

我有一个名为使用此方法的文件来获取它的实例：DatabaseHelper.java

public static DatabaseHelper getInstance(Context context) {
    if (mInstance == null) {
        mInstance = new DatabaseHelper(context.getApplicationContext());
    }
    return mInstance;
}

然后我有这样的方法（它在带有该错误的行中崩溃）。它几乎从不崩溃，但有时它确实如此。cursor.moveToFirst()

public Profile getProfile(long id) {
    SQLiteDatabase db = this.getReadableDatabase();

    String selectQuery = "SELECT * FROM " + TABLE_PROFILES + " WHERE " + KEY_PROFILES_ID + " = " + id;
    Cursor cursor = db.rawQuery(selectQuery, null);

    // looping through all rows and adding to list
    Profile profile = new Profile();
    if (cursor.moveToFirst()) {
        doWhatEver();
    }
    cursor.close();
    db.close();

    return profile;
}

就是这样，在我使用的所有方法中：

SQLiteDatabase db = this.getReadableDatabase(); (or Writable)

然后我关闭光标和数据库。在本例中，错误被扔到行中：

cursor.moveToFirst();

我不明白为什么错误说db关闭，如果我之前正在调用。请支持！谢谢:)this.getReadableDatabase()