I'm sorry you're running into this issue. Please see the attached updated file and it is opening fine in Google Web Designer now.
Here are the instruction to fix the file :
Open the author file in a text edit and search to see there's any id with "head" in it (id="head"). If there is, please rename head to head2, save it and the file can be open again in Google Web Designer.
Thanks
Sukhbir
(Google Web Designer Team)