when execute getReadableDatabase() error:getReadableDatabase() on a null object reference

when execute super.getReadableDatabase() show error:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference

the following is my code :

    public synchronized SQLiteDatabase getReadableDatabase() {
        return super.getReadableDatabase();
    }

    public List<AllCaseItem> getOtherPersonalCase(int personid) {

    Cursor c = null;
    List<AllCaseItem> myList = new ArrayList<AllCaseItem>();
    String selection="SELECT " + TABLE_CaseFolder + "." + CaseFolder_CaseId + "," +
            TABLE_CaseFolder + "." + CaseFolder_CaseCoverImageFileName + "," +
            TABLE_CaseFolder + "." + CaseFolder_CaseName + "," +
            TABLE_CaseFolder + "." + CaseFolder_CaseColor + "," +
            "COUNT(*)," + TABLE_CaseFolder + "." + CaseFolder_IsMedicalCase + "," +
            TABLE_CaseFolder + "." + CaseFolder_StartDate + "," +
            TABLE_CaseFolder + "." + CaseFolder_EndDate +
            " FROM " + TABLE_Event + " LEFT JOIN " + TABLE_CaseFolder +
            " ON " + TABLE_CaseFolder + "." + CaseFolder_CaseId + " = " + TABLE_Event + "." + CaseFolder_CaseId +
            " WHERE " + TABLE_Event + "." + Person_PersonId + " = " + personid +
            " AND " + TABLE_Event + "." + Event_IsDeleted + " = 0 " +
            " AND " + TABLE_CaseFolder + "." + CaseFolder_IsDeleted + " = 0" +
            " AND " + TABLE_CaseFolder + "." + Person_PersonId + " = 0" +
            " GROUP BY " + TABLE_Event + "." + CaseFolder_CaseId;
    try {
        synchronized (DBLOCK) {
            SQLiteDatabase db = this.getReadableDatabase();
            c = db.rawQuery(selection, null);
            if (c != null && c.moveToFirst()) {
                do {
                    AllCaseItem allCaseItem = new AllCaseItem();
                    allCaseItem.setCaseId(c.getInt(c.getColumnIndex(CaseFolder_CaseId)));
                    allCaseItem.setImageName(c.getString(c.getColumnIndex(CaseFolder_CaseCoverImageFileName)));
                    allCaseItem.setColor(c.getString(c.getColumnIndex(CaseFolder_CaseColor)));
                    allCaseItem.setCaseType(c.getString(c.getColumnIndex(CaseFolder_CaseName)));
                    allCaseItem.setStartDate(c.getLong(c.getColumnIndex(CaseFolder_StartDate)));
                    allCaseItem.setEndDate(c.getLong(c.getColumnIndex(CaseFolder_EndDate)));
                    allCaseItem.setIsMedicalCase(c.getInt(c.getColumnIndex(CaseFolder_IsMedicalCase)) == 1);
                    myList.add(allCaseItem);
                } while (c.moveToNext());
            }
        }
        c.close();
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    }
    return myList;
}

when execute SQLiteDatabase db = this.getReadableDatabase(); it is error