Description of problem:
Installator crashed when I tried to edit partition and set its mount point.

1. I went to installation destinastion where I selected all 3 disks
2. I chose to use blivet-gui for partitoning and clicked done
3. On sda I created 2 GiB /boot and 2 GiB swap
4. I created sda3 as raid0 using all disks. I used about half of all space, but I didn't fill mount point
5. I created sda4 as raid5 using all disks. I used the rest of the space and didn't set the mount point
6. Then I clicked on done which showed me error message that I didn't set root partition.
7. I right clicked on sda3 and chose to edit -> format.
8. I set mount point to / and clicked format.

Version-Release number of selected component:
anaconda-core-26.21.1-1.fc26.x86_64

The following was filed automatically by anaconda:
anaconda 26.21.1-1 exception report
Traceback (most recent call first):
  File "/usr/lib/python3.6/site-packages/blivetgui/dialogs/edit_dialog.py", line 205, in validate_user_input
    valid, msg = is_mountpoint_valid(self.mountpoints, selected_mnt, self.edit_device.format.mountpoint)
  File "/usr/lib/python3.6/site-packages/blivetgui/dialogs/edit_dialog.py", line 221, in run
    if not self.validate_user_input():
  File "/usr/lib/python3.6/site-packages/blivetgui/osinstall.py", line 174, in run_dialog
    response = dialog.run()
  File "/usr/lib/python3.6/site-packages/blivetgui/blivetgui.py", line 289, in format_device
    user_input = self.run_dialog(dialog)
AttributeError: 'SwapSpace' object has no attribute 'mountpoint'

Additional info:
addons:         com_redhat_kdump
cmdline:        /usr/libexec/system-python  /sbin/anaconda --liveinst --method=livecd:///dev/mapper/live-base
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         4.11.0-0.rc3.git0.2.fc26.x86_64
other involved packages: blivet-gui-2.1.1-1.fc26.noarch
product:        Fedora
release:        Fedora release 26 (Twenty Six)
type:           anaconda
version:        26