Could be this thing!
I have this in my app.yml
git clone https://github.com/merefield/discourse-topic-previews.git -b "theme_sidecar"
This is obviously an old repo with a special branch!
Will try with the latest one:
git clone https://github.com/paviliondev/discourse-topic-previews-sidecar
EDIT: It works!!!
Thanks a great deal to @raghukamath