New Post: Error when making a Tree definition

(Earlier I wrote that you can't use both manual and auto attachment. I was wrong. You actually can. That's why I've edited this post.)

As you use the ParentIdFilter and filter your items by PageId, I assume that your Lysdal.ConcertItem data type is actually a page data folder.

The page data folder always have PageId field that defaults to the page where this page data folder is attached.

With this:

<ElementStructure.AutoAttachments> 
    <NamedParent Name="Content" Position="Bottom" />
  </ElementStructure.AutoAttachments>

you don't attach the page data folder to any of the pages. So you get an error because the PageId is not set.

I recommend that you use a global data type instead of the page data folder with an explicit PageId field, which you'll be able to set.