Update for MoP/WoD: This may no longer be needed.

This addon tries to work around the Blizzard UI bug which results in the error message seen on forums regularly:

Blizzard_GuildUI\Blizzard_GuildRoster.lua:147: bad argument #3 to 'SetFormattedText' (string expected, got nil)
Blizzard_GuildUI\Blizzard_GuildRoster.lua:147: in function <...ace\AddOns\Blizzard_GuildUI\Blizzard_GuildRoster.lua:120>
<in C code>: in function `GuildRoster_Update'
Blizzard_GuildUI\Blizzard_GuildRoster.lua:70: in function <...ace\AddOns\Blizzard_GuildUI\Blizzard_GuildRoster.lua:56>

There is no configuration; simply install the addon. If you are not in a guild, the addon will not do anything.

Explanation of the bug (and the workaround), to the best of my knowledge:

When you click a name in the guild roster, a small "details" window pops up to one side. It goes away when you close the entire guild roster, but that name is still considered to be "selected". Even if you close just the details window on its own, the name still counts as "selected". This is a little odd, but it means that the next time you open the guild roster, the same name is already selected. So, this is normal.
When you cross between servers (zoning in/out of an instance, between continents, etc), guild information is not instantly available. This is normal.
Coming out of a load screen, your game client decides to update its cached data about your guild (GUILD_ROSTER_UPDATE event). This is normal. When it does this, it finds that there is a guild member that was "selected", and tries to fetch the additional information to display in the small details window. It does this even when your guild roster and details windows are both closed! This is... probably unwise.
Not all the guild information is present, so your client ends up with missing bits. The Blizzard UI tries to fill out the details window with missing bits, which results in the error.

The workaround: when starting to zone, we pretend that no guildmates have been selected. Several seconds after coming out of the load screen (at least as far as the UI knows - if you get stuck fighting the Blue Bar Boss, the UI may think it's finished anyhow), we stop pretending and go back to saying who's really selected. This means if you finish a load screen and immediately start clicking names on your guild roster, nothing will happen.

GuildRosterSelectedFix