What steps will reproduce the problem?
1. Join a party (was in a 5man party to run DTK)
2. After completing the instance, leave party
3. Target anything (including friendly NPCs/Hostile targets - not sure about targeting friendly Players)

What is the expected output? What do you see instead?
Expected to not throw an error every time I target something - instead I'm getting an error (see error log below)

What version of the product are you using?
v4.0.0-beta3-4-g444072c

Do you have an error log of what happened?
Ok, I checked my error log to see if there's any difference in each of the errors (because I got a new one EVERY time I targeted someone/something), and the difference in each error is in the "guid = 0xF130006938000062" section - the number is different each time, but anyway, here is a copy of the last error I received:
Date: 2009-08-14 01:40:46
ID: -1
Error occured in: Global
Count: 1
Message: frame = PitBull4_Groups_Party targetsUnitButton4; unit = nil; guid = 0xF130006938000062; is_shown = nil
...erface\AddOns\PitBull4\Modules\PowerBar\PowerBar.lua line 89:
   Usage: UnitPower("unit"[, type])
Debug:
   (tail call): ?
   ...erface\AddOns\PitBull4\Modules\PowerBar\PowerBar.lua:110 GetValue()
   ...terface\AddOns\PitBull4\ModuleHandling\BarModule.lua:40
      ...terface\AddOns\PitBull4\ModuleHandling\BarModule.lua:34
   ...terface\AddOns\PitBull4\ModuleHandling\BarModule.lua:263 UpdateFrame()
   PitBull4\ModuleHandling\Module.lua:288 Update()
   PitBull4\UnitFrame.lua:619 Update()
   PitBull4\UnitFrame.lua:645 UpdateGUID()
   PitBull4\Main.lua:1272 CheckGUIDForUnitID()
   PitBull4\Main.lua:1283 ?()
   ...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:146
      ...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:146
   [string "safecall Dispatcher[1]"]:4:
      [string "safecall Dispatcher[1]"]:4
   [C]: ?
   [string "safecall Dispatcher[1]"]:13: ?()
   ...ore\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:91 Fire()
   ...\AddOns\DataStore\libs\AceEvent-3.0\AceEvent-3.0.lua:119
      ...\AddOns\DataStore\libs\AceEvent-3.0\AceEvent-3.0.lua:118
   [C]: TurnOrActionStop()
   [string "TURNORACTION"]:4:
      [string "TURNORACTION"]:1
AddOns:
  Swatter, v3.1.14 (<%codename%>)
  AdvancedTradeSkillWindow, v
  AggroNotifier, v1.2.5
  Align, v
  Altoholic, v3.2.001b
  ArkInventory, v3.02
  AtlasLoot, vv5.07.04
  AtlasLootFu, vv5.07.04
  AucAdvanced, v5.6.4386 (KangaII)
  AucDb, v5.1.DEV.3583
  AucFilterBasic, v5.6.4386 (KangaII)
  AucFilterOutlier, v5.6.4386.2531
  AucMatchUndercut, v5.6.4386.2531
  AucScanData, v5.6.4386 (KangaII)
  AucStatClassic, v5.6.4386 (KangaII)
  AucStatHistogram, v5.6.4386 (KangaII)
  AucStatiLevel, v5.6.4386 (KangaII)
  AucStatPurchased, v5.6.4386 (KangaII)
  AucStatSales, v5.6.4386.2842
  AucStatSimple, v5.6.4386 (KangaII)
  AucStatStdDev, v5.6.4386 (KangaII)
  AucStatWOWEcon, v5.6.4386.2530
  AucUtilAHWindowControl, v5.6.4386.3311
  AucUtilAppraiser, v5.6.4386.2530
  AucUtilAskPrice, v5.6.4386.3175
  AucUtilAutoMagic, v5.6.4386.3142
  AucUtilCompactUI, v5.6.4386.2530
  AucUtilEasyBuyout, v5.6.4386.3583
  AucUtilItemSuggest, v5.6.4386.3108
  AucUtilPriceLevel, v5.6.4386.2545
  AucUtilScanButton, v5.6.4386.2530
  AucUtilScanFinish, v5.6.4386.2530
  AucUtilScanProgress, v5.6.4386.2530
  AucUtilSearchUI, v5.6.4386.3655
  AucUtilSimpleAuction, v5.6.4386.0
  AucUtilVendMarkup, v5.6.4386.2530
  Babylonian, v5.1.DEV.130
  Bartender4, v4.4.0-1-gcc58741
  BeanCounter, v5.6.4386 (KangaII)
  Broker2FuBar, v1.0
  Butsu, v1.10
  ButtonFacade, v3.2.275
  ButtonFacadeEntropy, v3.2.55
  ButtonFacadeLiteStep, v3.2.45
  ButtonFacadeNumein, v1.0
  ButtonFacadeOnyx, v3.2.55
  ButtonFacadeSleek, v3.2.0
  Carbonite, v3.21
  CarboniteTransfer, v1.01
  Cartographer, v2.0
  CartographerBattlegrounds, v2.0
  CartographerCoordinates, v2.0
  CartographerFoglight, v2.0
  CartographerGroupColors, v2.0
  CartographerGuildPositions, v2.0
  CartographerInstanceLoot, v2.0
  CartographerInstanceMaps, v2.0
  CartographerInstanceNotes, v2.0
  CartographerLookNFeel, v2.0
  CartographerNotes, v2.0
  CartographerPOI, v2.0
  CartographerWaypoints, v2.0
  CartographerZoneInfo, v2.0
  Chinchilla, vv2.0.8
  CloseUp, v3.2.001
  Configator, v5.1.DEV.130
  DataStore, v3.2.001
  DataStoreAchievements, v3.2.001
  DataStoreAuctions, v3.2.001
  DataStoreCharacters, v3.2.001
  DataStoreContainers, v3.2.001
  DataStoreCrafts, v3.2.001
  DataStoreCurrencies, v3.2.001
  DataStoreInventory, v3.2.001
  DataStoreMails, v3.2.001
  DataStorePets, v3.2.001
  DataStoreQuests, v3.2.001
  DataStoreReputations, v3.2.001
  DataStoreSkills, v3.2.001
  DataStoreSpells, v3.2.001
  DataStoreStats, v3.2.001
  DataStoreTalents, v3.2.001
  DBMCore, v
  DBMPartyWotLK, v
  DebugLib, v5.1.DEV.130
  Decursive, v2.4.2.1-2-ge913e35
  DropTheCheapestThing, vv3.4.1-1-gf4a35dd
  Enchantrix, v5.6.4386 (KangaII)
  EnchantrixBarker, v5.6.4386 (KangaII)
  Engravings, v3.1.0.13
  EnhancedColourPicker, vb1.9056.1
  EquipCompare, v2.16
  Examiner, v09.08.13
  Factionizer, v30200.1
  FriendsWithBenefits, v3.0.9.3
  FuBar, v
  FuBarCalendarFu, v3.0.0
  FuBarDualSpecFu, v2.0.0
  FuBarFactionsFu, v2.2.1-80
  FuBarLoginTimerFu, vv0.20-alpha
  FuBarRestFu, vv1.3.2
  FuBarSkillsPlusFu, v3.1.0
  FuBarTopScoreFu, vv1.0.5-4-g139baed
  FuBarTrainerFu, v0.2
  Gatherer, v3.1.14
  GathererDBWowhead, v1.0.2009-02-18
  Grid, v1.30200.2009081101
  HealBot, v3.2.0.1
  InCombatIndicator, v3.0.8
  InFlight, v3.2.001
  InFlightLoad, v3.2.001
  Informant, v5.6.4386 (KangaII)
  kgPanels, vr338
  LibRangeCheck20, v3.2
  LightHeaded, v297
  LightHeadedDataD, v297
  LightHeadedDataE, v297
  LightHeadedDataNPC, v297
  LightHeadedDataQIDNames, v297
  LinkWrangler, v1.79
  MikScrollingBattleText, v5.4.64
  numeinSharedMedia, v1.0
  oGlow, v1.12
  Omen, v3.0.8
  OmniCC, v2.5.6
  OPie, v
  Outfitter, v4.8b6
  PitBull4, vv4.0.0-beta3-4-g444072c
  Postal, v3.2.0
  Prat30, v3.2.4
  Prat30Libraries, v
  PratLinkHover, v1.0.2
  Quartz, v1.0
  QuartzFlight, v1.0
  QuartzFocus, v1.0
  QuartzGCD, v1.0
  QuartzInterrupt, v1.0
  QuartzLatency, v1.0
  QuartzMirror, v1.0
  QuartzPet, v1.0
  QuartzPlayer, v1.0
  QuartzRange, v1.0
  QuartzTarget, v1.0
  QuartzTradeskill, v1.0
  QuestItemBar, vr41
  RatingBuster, v
  Recount, v
  Reflux, v1.7
  SatrinaBuffFrame, v3.1
  Scorchio2, v
  SellOMatic, vv1.1.15
  SharedMedia, v3.0
  SimpleMarker, vv2.3.1-release
  SLDataText, v3.1.4
  SLDataTextHonor, v1.0
  SLDTExperience, v1.1.0
  SlideBar, v3.1.14 (<%codename%>)
  SmartRes, v6.03
  Squire, vr92
  Stubby, v5.6.4386 (KangaII)
  Talented, vr619
  TalentedSpecTabs, v1.0
  TipTac, v09.08.13
  TipTacItemRef, v09.08.13
  TipTacTalents, v09.08.13
  TradeskillInfo, v1.5.0
  TrinketMenu, v
  TrinketMenuButtonFacade, v1.0
  WGPlus, v1.2
  WIM, v3.1.2
  XPBarNone, vv1.4.6
  ZygorGuidesHorde, v1.0
  ZygorGuidesViewer, v1.0
  BlizRuntimeLib_enUS v3.2.0.30200 <us>
  (ck=1251)

Please provide any additional information below.
Once I did a /console reloadui, the errors stopped. I have not joined a party again to see if this bug will happen again.