Autosave Expanded Items of NSOutlineView doesn't work

I am trying to use the "Autosave Expanded Items" feature. When I expand a group with its children and restart the application all children are collapsed again and I don't know why they won't stay expanded. I'm using core data to store my source list items.

This is what I have done/set so far:

• Checked "Autosave Expanded Items" in NSOutlineView (Source List)
• Set a name for "Autosave"
• dataSource and delegate outlets assigned to my controller

This is my implementation for outlineView:persistentObjectForItem and outlineView:itemForPersistentObject.

- (id)outlineView:(NSOutlineView *)anOutlineView itemForPersistentObject:(id)object
{
    NSURL *objectURI = [[NSURL alloc] initWithString:(NSString *)object];  
    NSManagedObjectID *mObjectID = [_persistentStoreCoordinator managedObjectIDForURIRepresentation:objectURI]; 
    NSManagedObject *item = [_managedObjectContext existingObjectWithID:mObjectID error:nil];
    return item;  
}

- (id)outlineView:(NSOutlineView *)anOutlineView persistentObjectForItem:(id)item
{
    NSManagedObject *object = [item representedObject];
    NSManagedObjectID *objectID = [object objectID];
    return [[objectID URIRepresentation] absoluteString];
}

Any ideas? Thanks.

EDIT: I have a clue! The problem is maybe that the tree controller has not prepared its content on time. The methods applicationDidFinishLaunching, outlineView:persistentObjectForItem etc. are being be executed before the data has loaded or rather the NSOutlineView hasn't finished initializing yet. Any ideas how to solve this?

I've had the problem that my implementation of -outlineView:itemForPersistentObject: was not called at all. It turns out that this method is called when either "autosaveExpandedItems" or "autosaveName" is set. My solution was to set both properties in Code and NOT in InterfaceBuilder. When i set the properties after the delegate is assigned, the method gets called.