BUG ALERT IN SCRATCH 2.0 EDITOR
Recent updates to the Scratch editor prevent any project created using extension blocks to be saved and reopened. When reopening the project, all of the extensions blocks turn red and are labeled as undefined.
This is a known bug: https://scratch.mit.edu/discuss/topic/188451/?page=1#post-1856641
Let's hope the Scratch folks fix it soon!