Leapp preupgrade is failing with the error "Actor system_facts unexpectedly terminated with exit code: 1"

Issue

When performing an offline upgrade using Leapp with the DVD ISO, the leapp preupgrade process fails with the following error message:

# leapp preupgrade --no-rhsm --iso <iso-path>
..
Title: Actor system_facts unexpectedly terminated with exit code: 1
Summary: Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 74, in _do_run
    actor_instance.run(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 289, in run
    self.process(*args)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/actors/systemfacts/actor.py", line 57, in process
    self.produce(systemfacts.get_repositories_status())
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/actors/systemfacts/libraries/systemfacts.py", line 220, in get_repositories_status
    return RepositoriesFacts(repositories=repofileutils.get_parsed_repofiles())
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/repofileutils.py", line 74, in get_parsed_repofiles
    cmd = ['find', '-L'] + get_repodirs() + ['-maxdepth', '1', '-type', 'f', '-name', '*.repo']
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/repofileutils.py", line 54, in get_repodirs
    with dnf.base.Base() as base:
NameError: global name 'dnf' is not defined