Yes, I see this a lot too with my MailLooter addon. This was not an issue in previous ESO releases for a while... It was an issue before way over a year or two ago now.... It just started again with the DarkBrotherhood release.
I can reproduce it pretty easy with MailLooter. Just have only hireling mail in your inbox. Go to MailLooter and loot all hireling mail. Go back to the Inbox and Lua error...
Code:
EsoUI/Ingame/Mail/MailInbox_Shared.lua:51: attempt to index a nil value
stack traceback:
EsoUI/Ingame/Mail/MailInbox_Shared.lua:51: in function 'ZO_MailInboxShared_PopulateMailData'
EsoUI/Ingame/Mail/Keyboard/MailInbox_Keyboard.lua:425: in function 'MailInbox:OnMailReadable'
EsoUI/Ingame/Mail/Keyboard/MailInbox_Keyboard.lua:49: in function 'callback'
EsoUI/Libraries/Utility/ZO_CallbackObject.lua:111: in function 'ZO_CallbackObject:FireCallbacks'
EsoUI/Libraries/ZO_Scene/ZO_Scene.lua:248: in function 'ZO_Scene:SetState'
EsoUI/Libraries/ZO_Scene/ZO_Scene.lua:363: in function 'ZO_Scene:DetermineIfTransitionIsComplete'
EsoUI/Libraries/ZO_Scene/ZO_Scene.lua:306: in function 'ZO_Scene:OnSceneFragmentStateChange'
EsoUI/Libraries/ZO_Scene/ZO_SceneFragment.lua:108: in function 'ZO_SceneFragment:SetState'
EsoUI/Libraries/ZO_Scene/ZO_SceneFragment.lua:114: in function 'ZO_SceneFragment:OnShown'
EsoUI/Libraries/ZO_Scene/ZO_SceneFragmentTemplates.lua:57: in function '(anonymous)'
Deleting the last mail in the inbox from out code outside the inbox is the sure way to get this. But it happens other times as well. I think when the last select mail in the InBox is delete out from under the inbox.