Description of problem:
I simply clicked on sdb a External USB 3.0 hard drive on the left Disks selection window.

Version-Release number of selected component:
blivet-gui-1.0.2-1.fc23.noarch

Additional info:
reporter:       libreport-2.6.3
blivet-gui.log: 
cmdline:        /usr/bin/python3  /usr/bin/blivet-gui
executable:     /usr/bin/blivet-gui
hashmarkername: blivet-gui
kernel:         4.2.6-301.fc23.x86_64
product:        Fedora
release:        Fedora release 23 (Twenty Three)
type:           Python
version:        23

Truncated backtrace:
Traceback (most recent call last):
  File "/usr/lib/python3.4/site-packages/blivetgui/list_devices.py", line 191, in on_disk_selection_changed
    self.blivet_gui.update_partitions_view()
  File "/usr/lib/python3.4/site-packages/blivetgui/blivetgui.py", line 164, in update_partitions_view
    self.list_partitions.update_partitions_list(self.list_devices.selected_device)
  File "/usr/lib/python3.4/site-packages/blivetgui/list_partitions.py", line 76, in update_partitions_list
    if self._is_group_device(child):
  File "/usr/lib/python3.4/site-packages/blivetgui/list_partitions.py", line 125, in _is_group_device
    if luks_device.format and luks_device.format.type in ("lvmpv", "btrfs", "mdmember"):
AttributeError: 'NoneType' object has no attribute 'format'