Parent is the control that contains the listbox, not the window. Eg., if you place the listbox on a canvas, the canvas is the Parent of the listbox. If the listbox is just on the window, Parent is Nil (there is no containing control).
That said, Parent.Child syntax is specific to FolderItems. I think you’re confusing things here.
Depending on what your code is for, you might want to use TrueWindow.Title this will allow you to place the control on a ContainerControl or deeply nested…