TreeListModelCreateModelFunc

Prototype of the function called to create new child models when set_expanded is called.

This function can return null to indicate that item 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.

Parameters:

item	The item that is being expanded
user_data	User data passed when registering the function

Returns:

The model tracking the children of item or null if item can never have children