I had this issue when i do any pet battles, i dont get the normal ui interface and i figured it out to be this addon :(

10x Chatter\Modules\AutoPopup.lua:40 attempt to index local 'info' (a nil value)
Chatter\Modules\AutoPopup.lua:40 in function `AlwaysDecorate'
Chatter\Chatter-1.5.2.lua:196 in function <Chatter\Chatter.lua:183
(tail call): ?
CT_Core\CT_Core_Chat.lua:1177 in function `FCF_OpenTemporaryWindow'
Blizzard_PetBattleUI\Blizzard_PetBattleUI.lua:149 in function `PetBattleFrame_Display'
Blizzard_PetBattleUI\Blizzard_PetBattleUI.lua:95 in function <Blizzard_PetBattleUI\Blizzard_PetBattleUI.lua:92

Locals:
self = <table> {
 SetDefaultModuleLibraries = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:398
 Enable = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:325
 NewModule = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:266
 EnableModule = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:363
 modules = <table> {
 }
 GetModule = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:241
 IterateEmbeds = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:473
 SecureHookScript = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:410
 defaultModuleLibraries = <table> {
 }
 RawHookScript = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:395
 GetName = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:310
 name = "Chatter_Automatic Whisper Windows"
 IsEnabled = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:482
 orderedModules = <table> {
 }
 DisableModule = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:381
 baseName = "Chatter"
 ProcessWhisper = <function> defined @Chatter\Modules\AutoPopup.lua:48
 RegisterMessage = <function> defined @Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:116
 HookScript = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:366
 AlwaysDecorate = <function> defined @Chatter\Modules\AutoPopup.lua:21
 OnDisable = <function> defined @Chatter\Modules\AutoPopup.lua:14
 IsHooked = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:493
 OnEnable = <function> defined @Chatter\Modules\AutoPopup.lua:7
 Disable = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:345
 IsModule = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:248
 defaultModuleState = true
 SetDefaultModuleState = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:418
 UnregisterAllEvents = <function> defined @Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:200
 SendMessage = <function> defined @Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:85
 SetEnabledState = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:455
 Hook = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:274
 enabledState = false
 UnregisterMessage = <function> defined @Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:179
 RegisterEvent = <function> defined @Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:116
 UnregisterAllMessages = <function> defined @Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:200
 hooks = <table> {
 }
 SetDefaultModulePrototype = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:440
 SecureHook = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:336
 IterateModules = <function> defined @Ace3\AceAddon-3.0\AceAddon-3.0.lua:468
 UnhookAll = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:477
 RawHook = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:313
 Unhook = <function> defined @Ace3\AceHook-3.0\AceHook-3.0.lua:418
 moduleName = "Automatic Whisper Windows"
 UnregisterEvent = <function> defined @Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:179
 modName = "Automatic Whisper Windows"
}
frame = ChatFrame11 {
 0 = <userdata>
 RefreshIfNecessary = <function> defined =[C]:-1
 OnPostShow = <function> defined =[C]:-1
 GetNumVisibleLines = <function> defined =[C]:-1
 ScrollUp = <function> defined =[C]:-1
 SetTimeVisible = <function> defined =[C]:-1
 hasOwnFontObject = true
 OnFontObjectUpdated = <function> defined =[C]:-1
 resizeButton = ChatFrame11ResizeButton {
 }
 GetFading = <function> defined =[C]:-1
 Clear = <function> defined =[C]:-1
 shouldFadeAfterInact