These errors appear when right-clicking the Guidelime window to select a guide, or access it from Interface menu:

Message: Interface\FrameXML\InterfaceOptionsFrame.lua:423: attempt to index global 'InterfaceOptionsFrameTab1TabSpacer' (a nil value)
Time: Thu Apr 1 07:37:55 2021
Count: 1
Stack: Interface\FrameXML\InterfaceOptionsFrame.lua:423: attempt to index global 'InterfaceOptionsFrameTab1TabSpacer' (a nil value)
[string "@interface\FrameXML\InterfaceOptionsFrame.lua"]:423: in function InterfaceOptionsFrame_TabOnClick'
[string ":OnClick"]:3: in function <[string ":OnClick"]:1>
[string "=[C]"]: in function Click'
[string "@interface\FrameXML\InterfaceOptionsFrame.lua"]:476: in function InterfaceOptionsFrame_OpenToCategory'
[string "@interface\AddOns\Guidelime\Guidelime_Guides.lua"]:275: in function func'
[string "@interface\FrameXML\UIDropDownMenu.lua"]:960: in function `UIDropDownMenuButton_OnClick'
[string ":OnClick"]:1: in function <[string ":OnClick"]:1>

Message: Interface\FrameXML\InterfaceOptionsFrame.lua:158: attempt to index global 'InterfaceOptionsFrameCategoriesTop' (a nil value)
Time: Thu Apr 1 07:37:55 2021
Count: 1
Stack: Interface\FrameXML\InterfaceOptionsFrame.lua:158: attempt to index global 'InterfaceOptionsFrameCategoriesTop' (a nil value)
[string "@interface\FrameXML\InterfaceOptionsFrame.lua"]:158: in function InterfaceAddOnsList_Update'
[string "@interface\FrameXML\InterfaceOptionsFrame.lua"]:393: in function <Interface\FrameXML\InterfaceOptionsFrame.lua:389>
[string "=[C]"]: in function Show'
[string "@interface\FrameXML\InterfaceOptionsFrame.lua"]:204: in function InterfaceOptionsFrame_Show'
[string "@interface\FrameXML\InterfaceOptionsFrame.lua"]:487: in function InterfaceOptionsFrame_OpenToCategory'
[string "@interface\AddOns\Guidelime\Guidelime_Guides.lua"]:275: in function func'
[string "@interface\FrameXML\UIDropDownMenu.lua"]:960: in function UIDropDownMenuButton_OnClick'
[string ":OnClick"]:1: in function <[string ":OnClick"]:1>

See Stanzilla/WoWUIBugs#77 for possible fix ideas

Was a Blizzard issue. Has been fixed.