Leapp upgrade fails with "AttributeError: 'NoneType' object has no attribute 'path'"

Issue

• Leapp upgrade fails with the following error:

  Risk Factor: high (error)
  Title: Actor dnf_package_download 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/dnfpackagedownload/actor.py", line 55, in process
  target_iso=target_iso
    File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/dnfplugin.py", line 523, in perform_rpm_download
  target_iso=target_iso) as (context, overlay, target_repoids):
    File "/usr/lib64/python2.7/contextlib.py", line 17, in __enter__
  return self.gen.next()
    File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/dnfplugin.py", line 463, in _prepare_perform
      reserve_space = overlaygen.get_recommended_leapp_free_space(target_userspace_info.path)
  AttributeError: 'NoneType' object has no attribute 'path'