[eluser]WanWizard[/eluser]
MPTree extends CI_Model? And the call to parent::__construct() works? Is the database class loaded and properly defined?
The error is that $this->db is not assigned, which happens in CI_Model through a magic method. Does MPTree define a __get() by any chance that could interfere with that?
You're trying to get a 1.x library to work in 2.x, you might run into all kinds of issues...