TreeView no open/close

The top graph is the original code from this kind person GitHub - tootzoe/Ue5-Umg-Treeview who uses a TreeView to to traverse a file tree, I thought I’d repurpose it an swap out the whole file system thing with something basic, it’s supposed to look like this;

  1. < Collapsible
    —> A
  2. < Collapsible
    —> B
  3. < Collapsible
    —> C

Has it got something to do with ‘parent’ item, if so how would I fix this ?