Fix one at a time iterator to not check things like the platform

cura/OneAtATimeIterator.py

@@ -2,6 +2,7 @@
 # Cura is released under the terms of the AGPLv3 or higher.
 
 from UM.Scene.Iterator import Iterator
+from UM.Scene.SceneNode import SceneNode
 from functools import cmp_to_key
 from UM.Application import Application
 
@@ -10,13 +11,16 @@ from UM.Application import Application
 #  Take note that the list of nodes can have children (that may or may not contain mesh data)
 class OneAtATimeIterator(Iterator.Iterator):
     def __init__(self, scene_node):
-        super(OneAtATimeIterator, self).__init__(scene_node) # Call super to make multiple inheritence work.
+        super().__init__(scene_node) # Call super to make multiple inheritence work.
         self._hit_map = [[]]
         self._original_node_list = []
     
     def _fillStack(self):
         node_list = []
         for node in self._scene_node.getChildren():
+            if not type(node) is SceneNode:
+                continue
+
             if node.getBoundingBox().height > Application.getInstance().getMachineManager().getActiveProfile().getSettingValue("gantry_height"):
                 return
             if node.callDecoration("getConvexHull"):