As noted in the Github issue:
Did the following commands:
flatpak-builder --install-deps-from=flathub build-dir org.bforartists.Bforartists.json
flatpak-builder --repo=test_repo --force-clean build-dir org.bforartists.Bforartists.json
Receiving the following error:
Removing files/bin
Renaming bforartists.desktop to org.bforartists.Bforartists.desktop
Error: error while moving file $(pwd)/.flatpak-builder/rofiles/rofiles-yj7vGm/files/share/applications/bforartists.desktop: File or directory not found