I am trying to follow the top down action tutorial on the roblox wiki but it isnt working?

Ok, so I'm trying to learn how to code by following tutorials on the roblox wiki but I am running into some problems. here is the link to the page http://wiki.roblox.com/index.php?title=Top_Down_Action/Dialog

when I run my game I get this error 12:17:36.529 - DialogGui is not a valid member of ScreenGui

here is my code

local animationSpeedShow = .5
local animationSpeedHide = .5
local positionShow = UDim2.new(0.1, 0, 0, 0)
local positionHide = UDim2.new(0.1, 0, -0.2, 0)

local player = game.Players.LocalPlayer
local playerGui = game.StarterGui.DialogGui
local dialogFrame = playerGui.DialogGui.DialogFrame
local showDialog = game.ReplicatedStorage.ShowDialog

playerGui:SetTopbarTransparency(0)

local function onShowDialog(thumbnail, text, duration)
    local thumbnailPicture = dialogFrame:FindFirstChild(thumbnail)
    thumbnailPicture.Visible = true
    dialogFrame.TextLabel.Text = text
    dialogFrame:TweenPosition(positionShow, Enum.EasingDirection.Out, Enum.EasingStyle.Quint, animationSpeedShow)
    wait(duration)  
    dialogFrame:TweenPosition(positionHide, Enum.EasingDirection.Out, Enum.EasingStyle.Quint, animationSpeedHide)
    wait(animationSpeedHide)
    thumbnailPicture.Visible = false
end

showDialog.OnClientEvent:connect(onShowDialog)