attempt to index a field 'Character' (a nil value)?

On youtube a video that is called Christmas Special! | How to make an snowman boss. at the script that he used in video i always get an error. Can somone fix the problem?

local Humanoid = script.Parent:WaitForChild("Humanoid")
local HumanoidRootPart = script.Parent:WaitForChild("HumanoidRootPart")

local WalkRange = 20
local AcidBallSpeed = 50

local Closest = nil

spawn(function()
	while true do
		wait()
		if Closest ~= nil then
			HumanoidRootPart.BodyGyro.CFrame = CFrame.new(HumanoidRootPart.Position,Vector3.new(Closest.Character.HumanoidRootPart.Position.X,HumanoidRootPart.Position.Y,Closest.Character.HumanoidRootPart.Position.Z))
		end
	end
end)

while true do
	wait(2)
	local RandomValue = math.random(0,10)
	local List = {}
	
	for i,v in pairs(game.Players:GetChildren()) do
		if v and v.Character then
			local Distance = (HumanoidRootPart.Position - v.Character.HumanoidRootPart.Position).Magnitude
			table.insert(List,{v,Distance})
		end
	end
	
	table.sort(List,function(A,B)
		return A[2] <= B[2]
	end)
	if #List >= 1 then
		Closest = List[1][1]
	if RandomValue > 0 and RandomValue < 6 then
		local RandomWalkPosition = Vector3.new(math.random(-WalkRange,WalkRange),0,math.random(-WalkRange,WalkRange))
		Humanoid:MoveTo(HumanoidRootPart.Position+RandomWalkPosition)
	else
		local AcidBall = Instance.new("Part")
		AcidBall.Anchored = false
		AcidBall.Shape = "Ball"
		AcidBall.Size = Vector3.new(4,4,4)
		AcidBall.TopSurface = Enum.SurfaceType.Smooth
		AcidBall.BottomSurface = Enum.SurfaceType.Smooth
		
		AcidBall.Material = Enum.Material.Neon
		AcidBall.BrickColor = BrickColor.new("Lime green")
		AcidBall.CFrame = HumanoidRootPart.CFrame*CFrame.new(0,0,-10)
		AcidBall.Parent = workspace
		AcidBall.CanCollide = false
		
		local BV = Instance.new("BodyVelocity")
		BV.MaxForce = Vector3.new(100000,100000,100000)
		
		local Origin = HumanoidRootPart.Position
		BV.Velocity = (List[1][1].Character.HumanoidRootPart.Position - Origin).unit * AcidBallSpeed
		BV.Parent = AcidBall
		spawn(function()
			wait(1)
			AcidBall.Touched:Connect(function(Hit)
				if Hit and Hit.Parent and Hit.Parent:FindFirstChild("Humanoid") then
					Hit.Parent.Humanoid:TakeDamage(20)
				end
				AcidBall:Destroy()
				
			end)
		end)
		wait(2)
	      end
	end
end