I know the title is a little weird but I'll try and explain the problem.

Lets say you have a property from a currentPage that returns and id from a media node.

First you check if it's a folder.

def _is_folder( node ):
    return node.ContentType.Text == 'Folder

So far so good. Next what I want to do is loop all the children and return all the images

media = Media( some_id )

if _is_folder( media ):
    for item in media.Children:
        if item.ContentType.Text == 'Image':
            pass

This will throw an Object reference not set to an instance of an object on the ContentType property.

It's a little weird because when I check the type of 'item' it says it's a Media object. 

To fix this I can do:

for item in media.Children:
    item = Media( item.Id )

So in other words instantiate a new Media object using the Id of an other Media object. 

I find this weird and maybe it's a bug? I think I had the same with Node, that the nodes the .Children property returns aren't complete and are missing properties them self.