How to prevent indexing a nil value?

This script works in the sense that it makes all the right instances, but when I change the value of EXP to, say, 100, my player doesn't level up to level 2.

game.Players.PlayerAdded:connect(function(plr)
    local stats = Instance.new('IntValue', plr)
    stats.Name = 'leaderstats'
    experience = Instance.new('IntValue', stats)
    experience.Name = 'EXP'
    experience.Value = 0
    level = Instance.new('IntValue',stats)
    level.Name = 'Level'
    level.Value = 0
end)

 function level()
    local required=(level.Value*100)
    if experience.Value>=required then
        level.Value=level.Value+1
        experience.Value=experience.Value-required
    level()
wait(.01)
    end
end

    experience.Changed:connect(level)

I get the error Workspace.Script:22: attempt to index global 'experience' (a nil value) This means that somewhere on line 22 I went wrong. I don't see how, because experience is defined correctly and the fuction's name is level. Any help is appreciated, thank you for reading.

I would try getting the Value inside the player with a separate variable (Outside of the player added event). You may do this by looping through each player and checking to see if it has changed. Or if it's not a server script then don't use the loop and use local player instead!

for _, player in pairs (game.Players:GetChildren()) do
	local Stats = player:FindFirstChild'leaderstats'
	if Stats then
		local Experience = Stats:FindFirstChild'EXP'
		if Experience then
			Experience.Changed:connect(level)
		end
	end
end

OR if not a server script.

local player = game:GetService'Players'.LocalPlayer
local Stats = player:FindFirstChild'leaderstats'
if Stats then
	local Experience = Stats:FindFirstChild'EXP'
	if Experience then
		Experience.Changed:connect(level)
	end
end

ProTip: If you are adding values to a player then I would use the starter player service for that. Anything inside that will be copied to the player when they join.

Hope this helps! ;)