Description of problem:
I started the live installer, waited a few seconds, and this crash occurs reproducibly. 

In an earlier bug report (of which there are many) I found a clue it is caused by several devices with the same UUID, which I can confirm. 

For reference, this is the bug report that provided the clue:
https://bugzilla.redhat.com/show_bug.cgi?id=1472999
I note this live installer has (pip3 freeze) blivet package version:
blivet (3.0.0b1)
blivet-gui (2.1.8)
and the error occurs at this line:
https://github.com/storaged-project/blivet/blob/3.0-release/blivet/devicetree.py#L158
The relevant code (# added by me) being:
#        if newdev.uuid and newdev.uuid in [d.uuid for d in self._devices] and \
#           not isinstance(newdev, NoDevice):
#raise ValueError("device is already in tree")

These are the relevant RAID1 devices. Here is the output of blkid:

[liveuser@localhost-live ~]$ blkid
# non-relevant entries omitted
/dev/sdc1: UUID="12c3222d-2959-51ba-1d8f-0b64164d4fed" UUID_SUB="f4da13e4-d837-2836-20ac-3779cee2456c" LABEL="myraidname" TYPE="linux_raid_member" PARTUUID="25899521-7fdd-4731-b15f-891c64b8b1c5"
/dev/sdd1: UUID="12c3222d-2959-51ba-1d8f-0b64164d4fed" UUID_SUB="23fdef09-9ddd-392c-d605-6966ddd07101" LABEL="myraidname" TYPE="linux_raid_member" PARTUUID="25899521-7fdd-4731-b15f-891c64b8b1c5"
/dev/sde: UUID="7956d543-45c7-4344-9816-8cfc8a622355" UUID_SUB="e80a93a0-4732-4856-a164-1e277aea68a7" TYPE="btrfs"
/dev/sdf: UUID="7956d543-45c7-4344-9816-8cfc8a622355" UUID_SUB="b992d41e-aa97-4a38-8dfc-44c7cd72d8ae" TYPE="btrfs"
/dev/md127p1: LABEL="bigraid" UUID="55d77587-4737-45c4-a72c-6b343acff43b" TYPE="ext4" PARTLABEL="raidhome" PARTUUID="a6e13a91-6f14-4f6d-9ae7-750b5780dafc"

I note that while the UUID of RAID1 devices are identical (not deliberatly set by me, btw) the UUID_SUB is always unique. 
Perhaps blivet should add a device to the tree based on this value instead?
As far as I understand, RAID devices with the same UUID are not uncommon:
https://serverfault.com/a/205790

Here are all the related bug reports:
https://duckduckgo.com/?q=blivet+%22ValueError%3A+device+is+already+in+tree%22+site%3Aredhat.com&t=hj&ia=web

Version-Release number of selected component:
anaconda-core-28.22.10-1.fc28.x86_64

The following was filed automatically by anaconda:
anaconda 28.22.10 exception report
Traceback (most recent call first):
  File "/usr/lib/python3.6/site-packages/blivet/devicetree.py", line 158, in _add_device
    raise ValueError("device is already in tree")
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/populator/helpers/partition.py", line 112, in run
    self._devicetree._add_device(device)
  File "/usr/lib/python3.6/site-packages/blivet/populator/populator.py", line 263, in handle_device
    device = helper_class(self, info).run()
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/populator/populator.py", line 122, in _add_slave_devices
    self.handle_device(slave_info)
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/populator/helpers/mdraid.py", line 55, in run
    self._devicetree._add_slave_devices(self.data)
  File "/usr/lib/python3.6/site-packages/blivet/populator/populator.py", line 263, in handle_device
    device = helper_class(self, info).run()
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/populator/helpers/mdraid.py", line 216, in run
    self._devicetree.handle_device(array_info, update_orig_fmt=True)
  File "/usr/lib/python3.6/site-packages/blivet/populator/populator.py", line 303, in handle_format
    helper_class(self, info, device).run()
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/populator/populator.py", line 275, in handle_device
    self.handle_format(info, device)
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/populator/populator.py", line 462, in _populate
    self.handle_device(dev)
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/populator/populator.py", line 412, in populate
    self._populate()
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/blivet/blivet.py", line 161, in reset
    self.devicetree.populate(cleanup_only=cleanup_only)
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib64/python3.6/site-packages/pyanaconda/storage/osinstall.py", line 1670, in reset
    super().reset(cleanup_only=cleanup_only)
  File "/usr/lib/python3.6/site-packages/blivet/threads.py", line 53, in run_with_lock
    return m(*args, **kwargs)
  File "/usr/lib64/python3.6/site-packages/pyanaconda/storage/osinstall.py", line 2193, in storage_initialize
    storage.reset()
  File "/usr/lib64/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib64/python3.6/site-packages/pyanaconda/threading.py", line 291, in run
    threading.Thread.run(self)
ValueError: device is already in tree

Additional info:
addons:         com_redhat_kdump
cmdline:        /usr/bin/python3  /sbin/anaconda --liveinst --method=livecd:///dev/mapper/live-base
cmdline_file:   BOOT_IMAGE=/images/pxeboot/vmlinuz root=live:CDLABEL=Fedora-SciK-Live-28-1-1 rd.live.image quiet
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         4.16.3-301.fc28.x86_64
other involved packages: python3-libs-3.6.5-1.fc28.x86_64, python3-blivet-3.0.0-0.6.1.b1.fc28.noarch
product:        Fedora
release:        Fedora release 28 (Twenty Eight)
type:           anaconda
version:        28