After today's patch, the following error popped up.

Message: Interface\AddOns\GuildGreet\GuildGreet.lua:1228: attempt to call global 'GuildRoster' (a nil value)
Time: Wed Oct 14 07:25:24 2020
Count: 1
Stack: Interface\AddOns\GuildGreet\GuildGreet.lua:1228: attempt to call global 'GuildRoster' (a nil value)
[string "@interface\AddOns\GuildGreet\GuildGreet.lua"]:1228: in function GLDG_InitRoster' [string "@Interface\AddOns\GuildGreet\GuildGreet.lua"]:238: in function GLDG_OnEvent'
[string ":OnEvent"]:1: in function <[string ":OnEvent"]:1>

Locals: name = "GuildGreetFrameSettingsGreeting"
offline = true
(*temporary) = nil
(*temporary) = "attempt to call global 'GuildRoster' (a nil value)"
L =

{
Remove selection = "Remove selection"
List to chat when a player levels up (offline players, printed when you log in) = "List to chat when a player levels up (offline players, printed when you log in)"
achievment = "achievment"
Use local time instead of server time = "Use local time instead of server time"
Enable player context menu (may cause taint issues in raid frame, see tooltip) = "Enable player context menu (may cause taint issues in raid frame, see tooltip)"
SubTabDebug = "Debug"
Set guild collection = "Set guild collection"
has increased his level from %s to %s = "has increased his level from %s to %s"
No characters found = "No characters found"
Say see you later to guild and channel = "Say see you later to guild and channel"
Supress all = "Supress all"
Don't put players coming online on the greet list = "Don't put players coming online on the greet list"
At %s, %s promoted this player to rank %s. = "At %s, %s promoted this player to rank %s."
Custom collections = "Custom collections"
List grows upwards instead of downwards = "List grows upwards instead of downwards"
Printing information to chat = "Printing information to chat"
Say goodbye to guild and channel = "Say goodbye to guild and channel"
Selected collection = "Selected collection"
ChatMsg/The config string seems to be corrupted. Please generating a new one. = "The config string seems to be corrupted. Please generating a new one."
Display your own characters = "Display your own characters"
List alt and main names when player logs in = "List alt and main names when player logs in"
Current value = "Current value"
ChatMsg/Config string not found. = "Config string not found."
Using chat frame %d (%s) = "Using chat frame %d (%s)"
Write the config string = "Write the config string"
Only show level-up for levels above %d = "Only show level-up for levels above %d"
At %s, this character came online for the first time during this session. = "At %s, this character came online for the first time during this session."
Channel name to monitor = "Channel name to monitor"
Test trigger = "Test trigger"
(off) = " (off)"
Whisper greetings and grats to players = "Whisper greetings and grats to players"
Don't put players that get promoted on the greet list = "Don't put players that get promoted on the greet list"
coming online = "coming online"
Add main name to chat when an alt sends a message = "Add main name to chat when an alt sends a message"
Show /who request and response text in chat = "Show /who request and response text in chat"
later channel = "later channel"
List alt and main names when player logs off = "List alt and main names when player logs off"
GuildGreet = "GuildGreet"
Configuration options for displaying the players waiting for a greeting = "Configuration options for displaying the players waiting for a greeting"
Character reached level %s. = "Character reached level %s."
bye char = "bye char"
Clear greet list = "Clear greet list"
Greet guild when 'Greet Key' is pressed (also applies to saying bye) = "Greet guild when 'Greet Key' is pressed (also applies to saying bye)"
Create new collection = "Create new collection"
Collection %q = "Collection %q"
Only update guild roster on events = "Only update guild roster on events"
Whisper level up messages = "Whisper level up me

This likely has to do with them changing a lot of the api calls anymore.

GuildRoster(); has been changed to C_GuildInfo.GuildRoster()

i don't create the 9.0 Patch, becsuse i don't had access to the Beta.

If you can help, pls PR the fixes

It's all fixed, I think you are take the old LUA scripts.

See on your repository, the GuildGreet.lua and then the Line 1228. There stands C_GuildInfo.GuildRoster(); and not more GuildRoster();

update is up in trhe Client

https://www.curseforge.com/wow/addons/guild-greet-extended/files