Description

Alexander Alexandrov says :

While debugging I noticed another problem - the unit tests fail if you
don't override some of the properties from the Base<Object>

protected $tree_parent;

protected $topic_id;

Otherwise they get declared in the Base<Object> as

protected $tree_parent = 0;

protected $topic_id = 0;

and that breaks the getLevel() method. Perhaps it's worth to add this to
the wiki page.