You must set the Document property first.

If you are re-using this GoLayoutTree with the same document, you should Network to null or a new GoLayoutTreeNetwork.

If you are re-using the same GoLayoutTreeNetwork too, you should set the Roots properties, either by clearing it and letting FindRoots do its job, or by specifying all the GoLayoutTreeNodes that are the roots of your trees.

Reference

GoLayoutTree Class
GoLayoutTree Members