QuestionModel needs an _items default

[cahirodoherty-learningpool]

Subject of the issue/enhancement/features

Error thrown when a question component that expects items does not have any set

Steps to reproduce

Create a course with a Matching Question Component that does not have any items

Expected behaviour

The empty component should render and the course creator should recognise their mistake

Actual behaviour

Depending on extensions configuration, the course can fall over on page load

[moloko]

fixed in v4 by b9c5816 and v3 by 87820ae