The following happens on Emacs 28.0.50 (or when project.el 0.6.0 has
been installed via GNU ELPA).

Steps to reproduce:

  mkdir "some project"
  cd "some project"
  echo "foobar" > file.txt
  emacs -Q
  C-u C-x p g foo RET RET RET RET

Result:

  project--files-in-directory: File listing failed: find:
‘/home/jim/some’: No such file or directory
  find: ‘project/’: No such file or directory

I think the directory passed to `find' should be shell-quoted. See the
attached patch.