Attract: prevent rare error in ATTRACT_OT_open_meta_blendfile

blender_cloud/attract/__init__.py

@@ -668,6 +668,8 @@ class ATTRACT_OT_open_meta_blendfile(AttractOperatorMixin, Operator):
 
     @classmethod
     def poll(cls, context):
+        if context.selected_sequences is None:
+            return False
         return bool(
             any(cls.filename_from_metadata(s) for s in context.selected_sequences)
         )