It is nice that you got it working, however I still do not know why you got the error.
Before even running the query, the code checks whether the post type is hierarchical (true for page) and if the post type is public (true for page) and if the specific post has a parent (which is not true in all cases). Therefore it should not even be possible for the query to be run on creation of top level pages.
But please let me know if you run in to this problem again.
