Description of problem:
in manual partitioning, i set lvm for a mount point, enter 'modifiy' and removed the vg name, so i believe that it was left as <null>, then i entered 'modify' again and it crashed.
The following was filed automatically by anaconda:
anaconda 18.37.10 exception report
Traceback (most recent call first):
  File "/usr/lib64/python2.7/site-packages/pyanaconda/ui/gui/spokes/custom.py", line 2056, in run_vg_editor
    name=vg_name,
  File "/usr/lib64/python2.7/site-packages/pyanaconda/ui/gui/spokes/custom.py", line 2094, in on_modify_vg_clicked
    self.run_vg_editor(vg=vg, name=vg_name)
UnboundLocalError: local variable 'vg_name' referenced before assignment

Additional info:
cmdline:        /usr/bin/python  /sbin/anaconda
cmdline_file:   initrd=initrd.img inst.stage2=hd:LABEL=Fedora\x2018\x20x86_64 quiet BOOT_IMAGE=vmlinuz 
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         3.6.10-4.fc18.x86_64
other involved packages: 
package:        anaconda-18.37.10
product:        Fedora
release:        Cannot get release name.
type:           libreport
version:        18