Vega tree error key is null

I am trying to create a vega tree visualization. I am not sure how to complete the starting node.
I've tried to put null, -, but nothing works. If the parent of the last node is null I am getting error _._source.organizations.original.attributes.parent.objectGUID is null (that means that the key is null). If I put - then an error like missing key appears.
Can you suggest what to do, or what property should I change?
I am using vega tree based on an index.