fix tree print

Author: Licini

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Changed `SceneObject.frame` to read-only result of `Frame.from_transformation(SceneObject.worldtransformation)`, representing the local coordinate system of the scene object in world coordinates.
 * Changed `SceneObject.worldtransformation` to the multiplication of all transformations from the scene object to the root of the scene tree, there will no longer be an additional transformation in relation to the object's frame.
 * Fixed call to `astar_shortest_path` in `Graph.shortest_path`.
+* Fixed a bug when printing an empty `Tree`.
 
 ### Removed
 

src/compas/datastructures/tree/tree.py

@@ -465,7 +465,8 @@ def traverse(node, hierarchy, prefix="", last=True, depth=0):
             for i, child in enumerate(node.children):
                 traverse(child, hierarchy, prefix, i == len(node.children) - 1, depth + 1)
 
-        traverse(self.root, hierarchy)
+        if self.root:
+            traverse(self.root, hierarchy)
 
         return "\n".join(hierarchy)