Server Script in Workspace:

game.Players.PlayerAdded:connect(function(plr)
    local value = Instance.new("StringValue")
    value.Value = plr.Name
    value.Parent = game.Lighting.PlayerStore
    script.Trigger:FireAllClients(plr)
end)

game.Players.PlayerRemoving:connect(function(plr)
    if game.Lighting.PlayerStore:FindFirstChild(plr.Name) then
        game.Lighting.PlayerStore[plr.Name]:Remove()
    end
    script.Trigger2:FireAllClients(plr)
end)

LocalScript in StarterGui under a ScreenGui

local localplayer = game.Players.LocalPlayer
local currentSlot = 0
local back = script.Parent.Back
local players = game.Lighting.PlayerStore:GetChildren()

local function updatelist()
    for i,v in pairs(back:GetChildren()) do
        v.Position = UDim2.new(.02, 0, 0, (35*currentSlot))
    end
    back.Size = UDim2.new(.2, 0, (.048 * currentSlot), 0)
end

game.Workspace.Script.Trigger.OnClientEvent:connect(function(plr)
    local player = plr.Name

    if player ~= localplayer.Name then
        local default = script:WaitForChild("Default"):Clone()
        default.Parent = back
        default.PlayerName.Text = player
        default.Name = player
        default.Slot.Value = currentSlot
        default.Position = default.Position + UDim2.new(0, 0, 0, (35 * currentSlot))
        back.Size = back.Size + UDim2.new(0, 0, (.048 * currentSlot), 0)
        currentSlot = currentSlot + 1
    elseif player == localplayer.Name then
        for i,v in pairs(players) do
            local default = script:WaitForChild("Default"):Clone()
            default.Parent = back
            default.PlayerName.Text = v.Value
            default.Name = v.Value
            default.Slot.Value = currentSlot
            default.Position = default.Position + UDim2.new(0, 0, 0, (35 * currentSlot))
            back.Size = back.Size + UDim2.new(0, 0, (.048 * currentSlot), 0)
            currentSlot = currentSlot + 1
        end     
    end
end)

game.Workspace.Script.Trigger2.OnClientEvent:connect(function(plr)
    local player = plr.Name --Here it errors, it says plr is nil?
    local slot
    for i,v in pairs(back:GetChildren()) do
        if v.Name == player then
            slot = v.Slot.Value
            v:Destroy()
        end
    end
    if slot ~= nil then
        for i,v in pairs(back:GetChildren()) do
            if v.Slot.Value > slot then
                v.Slot.Value = v.Slot.Value - 1
            end
        end
    end
    currentSlot = currentSlot - 1
    slot = nil
    updatelist()
end)

I am making a custom PlayerList, and for some reason, it returns player as nil (I put a note next to where it is erroring at)

Help is appreciated, thanks! :D