I created a new Container implementation, MockupContainer.
MockupContainer is a container that implements Container.Indexed and Container.Hierarchical. The contents of the container are generated at runtime, and can be customized. The container allows some parametrisation: item count, property count, number of children and the delay can easily be changed. The delay affects the time it takes to retrieve items, or item related attributes (such as the parent and children of an item).
MockupContainer could be used for example in UI-mockups, or for testing components that use a datasource.
Thanks for testing. The problem Joonas faced is probably
#3165 . The nullpointer was a) lazy coding b) b) not obeying the contract for getItem(). Those should be fixed now (added workaround for 3165).