Description of problem:
dnf subcommands group list and mark are returning errors

Version-Release number of selected component (if applicable):
dnf-1.0.2-2.fc23.1.noarch

How reproducible:
Always

Steps to Reproduce:
Just run:
1.1. Mark: #dnf group mark virtualization
or...
1.2. List: #dnf group list

Actual results:
1.1. Mark
[root@localhost ~]# dnf group mark virtualization
Last metadata expiration check performed 0:16:27 ago on Wed Jul 22 23:23:24 2015.
Groups marked installed: Virtualization
[Errno 2] No such file or directory: '/var/lib/dnf/groups.json'

1.2. List
[root@localhost ~]# dnf group list
Last metadata expiration check performed 0:13:38 ago on Wed Jul 22 23:23:24 2015.
Traceback (most recent call last):
  File "/usr/bin/dnf", line 35, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 190, in user_main
    errcode = main(args)
  File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 84, in main
    return _main(base, args)
  File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 139, in _main
    cli.run()
  File "/usr/lib/python3.4/site-packages/dnf/cli/cli.py", line 1076, in run
    return self.command.run(self.base.extcmds)
  File "/usr/lib/python3.4/site-packages/dnf/cli/commands/group.py", line 383, in run
    return self._list(extcmds)
  File "/usr/lib/python3.4/site-packages/dnf/cli/commands/group.py", line 167, in _list
    for group in userlist:
TypeError: 'NoneType' object is not iterable

Expected results:
1.1. just mark group of packages to be installed;
1.2. list group of packages available

Additional info:
Linux localhost.localdomain 4.2.0-0.rc3.git1.1.fc23.x86_64 #1 SMP Tue Jul 21 13:09:50 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Used image: Fedora-Live-Xfce-x86_64-23-20150722.iso