is there a fix for Nil value for local user - lua

Question

This is for a phone script ive been working on and im trying to update some code but I have been getting a single error.
Error #phone/server.lua:37: attempt to index a nil value (local 'user')
ref (#phone/server.lua:37)
handler (#framework/server/main.lua:242)
getNumberPhone (#phone/server.lua:36)
handler (#phone/server.lua:268)
I tried the one way I was shown here before with no luck.
line 37
function getNumberPhone(source, n)
local n = 0
TriggerEvent('f:getPlayer', source, function(user)
n = user.getPhoneNumber()
end)
return n
end
line 242
AddEventHandler("f:getPlayer", function(user, cb)
if not cb then return end
if(Users)then
if(Users[user])then
cb(Users[user])
else
cb(nil)
end
else
cb(nil)
end
end)
line 36
function getNumberPhone(source, n)
local n = 0
TriggerEvent('f:getPlayer', source, function(user)
n = user.getPhoneNumber()
end)
return n
end
line 286
RegisterServerEvent('gcPhone:allUpdate')
AddEventHandler('gcPhone:allUpdate', function()
local source = source
local identifier = GetPlayerIdentifiers(source)[1]
TriggerClientEvent("gcPhone:myPhoneNumber", source, getNumberPhone(source))
TriggerClientEvent("gcPhone:allMessage", source, getMessages(identifier,source))
TriggerClientEvent("gcPhone:contactList", source, getContacts(identifier))
end)

Answer 1

Explaining the Error
In Lua, this is one of the most common errors you will run into, so it is very important for you to know how to solve it.
The error: attempt to index a nil value
To understand what this error means, you only need to understand these concepts.
In Lua, only tables can be indexed (i.e. myTable[myIndex])
In Lua, if a variable evaluates to nil, then attempting to index it as if it were a table throws an error
So it should be a little easier to understand the error you received. A more explicit way to describe this error would be something like "the Lua interpreter attempted to index your variable user on line 37 but user evaluated to nil."
Your Specific Case
On line 242 you are calling a callback and passing nil
cb(nil)
This callback sends this nil value to line 37 as the user parameter
TriggerEvent('f:getPlayer', source, function(user)
n = user.getPhoneNumber()
end)
So when you try to run user.getPhoneNumber() you are actually running nil.getPhoneNumber(), which throws the error you saw.
Ways to fix this type of error
1) Whenever you are working with a variable that could be nil, create an if statement checking to see if it is nil before proceeding.
2) Make sure you never set that variable to nil.
Ways to fix your specific error
1) On line 36, create this if statement
TriggerEvent('f:getPlayer', source, function(user)
if user ~= nil then
n = user.getPhoneNumber()
end
end)
Or do a similar nil check like this
TriggerEvent('f:getPlayer', source, function(user)
if user then
n = user.getPhoneNumber()
end
end)
2) Always pass a user in your callback. For example, on line 242 and elsewhere, pass an actual user object.
cb(Users[someUser])