Android: Cannot perform this operation because the connection pool has been closed

Remove

db.close();

If you try another operation after closing the database, it will give you that exception.

The documentation says:

Releases a reference to the object, closing the object…

Also, check out
Android SQLite closed exception about a comment from an Android Framework engineer which states that it is not necessary to close the database connection, however this is only when it is managed in a ContentProvider.