Android查询数据库的时候，cursor报错空指针，数据库有表也有数据，求大神指点

[歆鸢]

public ArrayList doQuery(){
Cursor cursor =dbHelper.getWritableDatabase().query("notes", null, null, null, null, null, null);// 这里报错空指针
ArrayList ls = new ArrayList();

if (cursor.moveToFirst()) {
do {
int id = cursor.getInt(cursor.getColumnIndex("id"));
String title = cursor.getString(cursor.getColumnIndex("title"));
ls.add(title);
} while (cursor.moveToNext());
}
cursor.close();
return ls;
}