This content type parent must be a Decision Tree node & Fatal error: Call to protected method DecisionMaker::display_decision_form() from context 'CGDecisionTree'

I'm trying to test DecisionMaker but can't get it to work at all.

The documentation says that the Decision Tree Content Type (CT) as well ass the
Decision List CT get their questions from their children question content types.

So when i create a DecisionTree CT and a child "Decision node (Qeustion)" I get
the error "This content type parent must be a Decision Tree node". I also cannot
add a Decision node (Questino) to the root. The only way I can add any question
or Decision Node (Qestion) is by making it a child of a "Decision list".

Doing so (making Decision Node a child of a Decision List) will generate the
error "Fatal error: Call to protected method
DecisionMaker::display_decision_form() from context 'CGDecisionTree' in
.../modules/DecisionMaker/contenttype.decisiontree.php on line 136" in the
output of the page.

I didn't do anything special yet just added simple options:
Female|f
Male|m
(that's the options in their entirety).

Could you update the documentation if I'm doing something wrong and check what's
going on here with the fatal error?