This error occurs upon loading into a new zone, or when reloading the UI. This is probably an error just related to the new cross-realm button added on the quest tracker:
Lua Code:
3x nibRealUI\Misc\Misc.lua:44: attempt to index field 'PARTY_INVITE_XREALM' (a nil value)
nibRealUI\Misc\Misc.lua:44: in main chunk