object -- a g:object instance with the item that is being expanded
result -- a g:list-model object tracking the children of object or nil if object can never have children

Prototype of the function called to create new child models when the gtk:tree-list-row-expanded function is called. This function can return nil to indicate that object is guaranteed to be a leaf node and will never have children. If it does not have children but may get children later, it should return an empty model that is filled once children arrive.