WowAce

Message: ..\AddOns\RangeDisplay\RangeDisplay.lua line 455:
attempt to call method 'setDisplayColor' (a nil value)
Debug:
[string "=[C]"]: setDisplayColor()
[string "@RangeDisplay\RangeDisplay.lua"]:455: update()
[string "@RangeDisplay\RangeDisplay.lua"]:479:
RangeDisplay\RangeDisplay.lua:474

Message: ..\AddOns\RangeDisplay\RangeDisplay.lua line 262:
attempt to call method 'setDisplayColor' (a nil value)
Debug:
[string "=(tail call)"]: ?
[string "@RangeDisplay\RangeDisplay.lua"]:262: targetChanged()
[string "@RangeDisplay\RangeDisplay.lua"]:800:
RangeDisplay\RangeDisplay.lua:799
[string "=[C]"]: ?
[string "@Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:29:
...Ons\Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:25
[string "@Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:64:

For developers, see https://github.com/Stanzilla/WoWUIBugs/wiki/9.0.1-Consolidated-UI-Changes#backdrop-system-changes

For RangeDisplay.lua

Change 501 to:

    ud.mainFrame = CreateFrame("Frame", "RangeDisplayMainFrame_" .. unit, UIParent, BackdropTemplateMixin and "BackdropTemplate")

 Change 510 to:

    ud.rangeFrame = CreateFrame("Frame", "RangeDisplayFrame_" .. unit, ud.mainFrame, BackdropTemplateMixin and "BackdropTemplate")

 Change 899 to:

    self.dummyOpts = CreateFrame("Frame", AppName .. "DummyOptions", UIParent, BackdropTemplateMixin and "BackdropTemplate")

It looks like the developer has updated for 9.0.1; you can close this ticket.