Thank you for sharing your code, looks good!
Took me while to look over it (needed to set up a new elementary OS virtual machine), downloaded your code, built it, and indeed got the same error when trying to install it.
That error led me to look into your
post_install.py file, and noticed a typo on the first line (read more about shebang), a space had made its way where no space should be present:
After changing it to
#!/usr/bin/env python3, deleting the build folder, and running the build & install commands again, the schema was parsed successfully:
And the app runs as expected:
Vala is a nice programming language, and I wish you great success in your learning process. Never hesitate to ask for help when needed, and help others when you can.