Description of problem:
When I try anaconda from a USB key and I follow the usual steps I get into the
bootloader dialog with the wrong partition to boot into (USB key), with the
dropdown box disabled (I cannot change it), and if I try to go back I get into
the dialog where I select the "default partition" except that now the drop down
menu is disabled (also), and when trying to go to the next screen I get the
following:

 (snipped)
 /iw/autopart_type.py, line 73, in getNext
 /gui.py, line 1034, in nextClicked
 TypeError: iter must be a GtkTreeIter

Version-Release number of selected component (if applicable):
This is in f7t3.

Steps to Reproduce:
1. Try anaconda bro a bootable USB memory stick
2. Disable the USB key for installation (/dev/sda)
3. Select the internal HDD as the default and only device (SATA) (/dev/sdb)
4. [The bootloader dialog should have a greyed out dropdown menu with /dev/sda]
5. Go back
6. [The default device dialog should have a greyed out dropdown menu with nothing]
7. Go forward
8. Crash