Created attachment 1611037 [details]
report.zip

Steps:

a) Download Fedora-Workstation-Live-x86_64-Rawhide-20190902.n.0.iso and boot it in a virtual machine which already contains another Fedora.
b) after it boots, click "Install Fedora"
c) pick English/English and "I want to proceed"
d) pick the destination and there "Custom" (not "Advanced Custom (Blivet-GUI)")
e) the LVM is preselected in the combo, left it as is, and delete all existing partitions from the disk
f) then click the "+" and use:
   Mount Point: /
   Desired Capacity: 19.5GB
g) click Add mount point

The anaconda setup crashes. The report it provided is attached (zipped, it's too large). The top of it says:

anaconda 32.3 exception report
Traceback (most recent call first):
  File "/usr/lib64/python3.8/site-packages/pyanaconda/ui/gui/spokes/lib/custom_storage_helpers.py", line 280, in on_add_confirm_clicked
    self._warningLabel.set_text(self._error)
TypeError: Argument 1 does not allow None as a value

Local variables in innermost frame:
self: <pyanaconda.ui.gui.spokes.lib.custom_storage_helpers.AddDialog object at 0x7f8a96be1f70>
button: <Gtk.Button object at 0x7f8a9416fd00 (GtkButton at 0x55bbe173ca40)>
args: ()