Guard better against RaidChooser's selected_level being None

This happens more often since blivet dropped linear from mdraid
in storaged-project/blivet#1236 . It was
the only RAID level supported for mdraid that had min_members of
1. When you add the first member for a new mdraid set, of course,
there is only one member.

RaidChooser's update method checks the `min_members` on each
level and only adds it to the liststore of available levels if we
currently have at least that many members. So, before that blivet
change, when there was only one member, "linear" was the only
level in the liststore, and was necessarily the selected_level.
Now linear is gone, when there is only one member of an mdraid
set, there are *no* levels in the liststore, which means
selected_level winds up being None.

There are three places I can see in add_dialog.py which don't
guard against selected_level being None. This fixes each of them.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

blivetgui/dialogs/add_dialog.py

@@ -562,11 +562,12 @@ def raid_member_max_size(self):
 
  device_type = self.selected_type
  num_parents = self._get_number_selected_parents()
+ level = self._raid_chooser.selected_level
 
  if device_type not in self.supported_raids.keys() or num_parents == 1:
  return (False, None)
 
- elif self._raid_chooser.selected_level.name in ("linear", "single"):
+ elif level and level.name in ("linear", "single"):
  return (False, None)
 
  else:
@@ -863,7 +864,8 @@ def add_advanced_options(self):
  self.advanced.destroy()
 
  if device_type in ("lvm", "lvmvg", "partition", "mdraid"):
- if device_type == "mdraid" and self._raid_chooser.selected_level.name == "raid1":
+ level = self._raid_chooser.selected_level
+ if device_type == "mdraid" and level and level.name == "raid1":
  self.advanced = None
  self.widgets_dict["advanced"] = []
  else:
@@ -1068,7 +1070,9 @@ def get_selection(self):
  encryption_selection = self._encryption_chooser.get_selection()
 
  if device_type in ("btrfs volume", "mdraid", "lvmlv"):
- raid_level = self._raid_chooser.selected_level.name
+ raid_level = self._raid_chooser.selected_level
+ if raid_level:
+ raid_level = raid_level.name
  else:
  raid_level = None