Fix poll func of CYCLES_OT_use_shading_nodes.

Operators' poll func might be called from anywhere in Blender, so they should
not make any assumption about available context. material, lamp and world
are specific to context from Properties space...

intern/cycles/blender/addon/ui.py

@@ -629,7 +629,8 @@ class CYCLES_OT_use_shading_nodes(Operator):
 
     @classmethod
     def poll(cls, context):
-        return context.material or context.world or context.lamp
+        return (getattr(context, "material", False) or getattr(context, "world", False) or
+                getattr(context, "lamp", False))
 
     def execute(self, context):
         if context.material: