Description of problem:
This error popped up after adding / partition while not speciying amount of disk space to be reserved.
The following was filed automatically by anaconda:
anaconda 19.16-1 exception report
Traceback (most recent call first):
  File "/usr/lib64/python2.7/site-packages/pyanaconda/ui/gui/spokes/custom.py", line 1674, in on_add_clicked
    if dialog.size.convertTo(spec="mb") < 1:
AttributeError: 'NoneType' object has no attribute 'convertTo'

Version-Release number of selected component:
anaconda-19.16-1

Additional info:
cmdline:        /usr/bin/python  /sbin/anaconda
cmdline_file:   initrd=initrd.img inst.stage2=hd:LABEL=Fedora\x2019-Alpha\x20x86_64 quiet BOOT_IMAGE=vmlinuz 
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         3.9.0-0.rc4.git0.1.fc19.x86_64
product:        Fedora
release:        Cannot get release name.
type:           anaconda
version:        19-Alpha

Truncated backtrace:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/pyanaconda/ui/gui/spokes/custom.py", line 1674, in on_add_clicked
    if dialog.size.convertTo(spec="mb") < 1:
AttributeError: 'NoneType' object has no attribute 'convertTo'