Can't build

[junocomp]

I've adjusted the .json file so it copies gtk-3.0 instead of 3.22. I keep getting this error everytime I try to build.

flatpak-builder .build org.gtk.Gtk3theme.elementary.json --install
Downloading sources
Initialized empty Git repository in /home/caligaris/org.gtk.Gtk3theme.elementary/.flatpak-builder/git/git_github.com_elementary_stylesheet.git-BEZIPZ/
Fetching git repo git://github.com/elementary/stylesheet.git, ref refs/heads/master
remote: Enumerating objects: 77, done.
remote: Counting objects: 100% (77/77), done.
remote: Compressing objects: 100% (48/48), done.
remote: Total 77 (delta 36), reused 47 (delta 24), pack-reused 0
Unpacking objects: 100% (77/77), done.
From git://github.com/elementary/stylesheet
 * [new branch]      master     -> master
Initializing build dir
Committing stage init to cache
Starting build of org.gtk.Gtk3theme.elementary
========================================================================
Building module elementary in /home/caligaris/org.gtk.Gtk3theme.elementary/.flatpak-builder/build/elementary-1
========================================================================
Already on 'master'
Running: install -dm755 /usr/share/runtime/share/themes/elementary/gtk-3.0
Running: cp -aL gtk-3.0/* /usr/share/runtime/share/themes/elementary/gtk-3.0
cp: cannot stat 'gtk-3.0/*': No such file or directory
Error: module elementary: Child process exited with code 1

I know I can install this theme from Flathub, but I rather learn how to bundle my own theme.