Installing on Linux, I found two problems.
1) the Makefile CFLAGS includes -DDATADIR=\"$(datadir)\" It should be -DDATADIR=\"$(pkgdatadir)\"
2) the function gtk_builder_add_from_file() is called two places in gui.c. The function returns zero on error, and non-zero on success. The code as written hopes expects 1 on success.
Oddly enough, the two errors counteract one another when run with BRP_PACU.ui in the current working directory.