1x Interface/AddOns/RaiderIO/core.lua:5240: attempt to index global 'WhoListScrollFrame' (a nil value)
[string "@Interface/AddOns/RaiderIO/core.lua"]:5240: in function OnLoad' [string "@Interface/AddOns/RaiderIO/core.lua"]:601: in function Load'
[string "@Interface/AddOns/RaiderIO/core.lua"]:3912: in function <Interface/AddOns/RaiderIO/core.lua:3904>
[string "@Interface/AddOns/RaiderIO/core.lua"]:3944: in function callbackFunc' [string "@Interface/AddOns/RaiderIO/core.lua"]:836: in function SendEvent'
[string "@Interface/AddOns/RaiderIO/core.lua"]:771: in function <Interface/AddOns/RaiderIO/core.lua:767>

Locals:
self =

{
index = 9
Enable = defined @Interface/AddOns/RaiderIO/core.lua:621
GetDependencies = defined @Interface/AddOns/RaiderIO/core.lua:663
OnDisable = defined @Interface/AddOns/RaiderIO/core.lua:695
HasDependencies = defined @Interface/AddOns/RaiderIO/core.lua:646
OnLoad = defined @Interface/AddOns/RaiderIO/core.lua:5238
SetLoaded = defined @Interface/AddOns/RaiderIO/core.lua:591
IsLoaded = defined @Interface/AddOns/RaiderIO/core.lua:611
enabled = true
loaded = true
id = "WhoTooltip"
OnEnable = defined @Interface/AddOns/RaiderIO/core.lua:691
Disable = defined @Interface/AddOns/RaiderIO/core.lua:631
SetDependencies = defined @Interface/AddOns/RaiderIO/core.lua:641
IsEnabled = defined @Interface/AddOns/RaiderIO/core.lua:616
SetEnabled = defined @Interface/AddOns/RaiderIO/core.lua:606
Load = defined @Interface/AddOns/RaiderIO/core.lua:596
CanLoad = defined @Interface/AddOns/RaiderIO/core.lua:680
}
(*temporary) = defined =[C]:-1
(*temporary) = nil
(*temporary) = true
(*temporary) = {
index = 9
Enable = defined @Interface/AddOns/RaiderIO/core.lua:621
GetDependencies = defined @Interface/AddOns/RaiderIO/core.lua:663
OnDisable = defined @Interface/AddOns/RaiderIO/core.lua:695
HasDependencies = defined @Interface/AddOns/RaiderIO/core.lua:646
OnLoad = defined @Interface/AddOns/RaiderIO/core.lua:5238
SetLoaded = defined @Interface/AddOns/RaiderIO/core.lua:591
IsLoaded = defined @Interface/AddOns/RaiderIO/core.lua:611
enabled = true
loaded = true
id = "WhoTooltip"
OnEnable = defined @Interface/AddOns/RaiderIO/core.lua:691
Disable = defined @Interface/AddOns/RaiderIO/core.lua:631
SetDependencies = defined @Interface/AddOns/RaiderIO/core.lua:641
IsEnabled = defined @Interface/AddOns/RaiderIO/core.lua:616
SetEnabled = defined @Interface/AddOns/RaiderIO/core.lua:606
Load = defined @Interface/AddOns/RaiderIO/core.lua:596
CanLoad = defined @Interface/AddOns/RaiderIO/core.lua:680
}
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index global 'WhoListScrollFrame' (a nil value)"
OnEnter = defined @Interface/AddOns/RaiderIO/core.lua:5206
OnLeave = defined @Interface/AddOns/RaiderIO/core.lua:5223
OnScroll = defined @Interface/AddOns/RaiderIO/core.lua:5230

Based on the error, it appears you are using a bit older version of the addon, that doesn't contain the 10.0.0 patch changes.

If you update the addon it should get rid of this message for you. Just ensure you update using the latest regular release, and not a alpha or beta version, those can right now be some weeks or months old as they aren't frequent.