Codeigniter: Having error on Model query

I keep having this error: Call to a member function query() on a non-object Anytine I try to make query on model, And I found how to fix the issue:

1. Edit file application/config/autoload.php line 55:

- $autoload['libraries'] = array();
+ $autoload['libraries'] = array('database', 'session');

2. Edit file application/config/config.php line 228 by editing key

- $config['encryption_key'] = '';
+ $config['encryption_key'] = '123456789'; // add ur key here

And it works!