Why did it give me an error saying: attempt to index nil with 'PlayerGui'?

The cause of the error could be found at line 14

local seat = script.Parent
local sound = script.Parent.On
local off = script.Parent.Off
local player = game.Players.LocalPlayer
local ui = script.Parent:FindFirstChild("PoliceGUI")

seat.Changed:Connect(function(ch)
	if ch == "Occupant" then
		if seat.Occupant then
			local player = game.Players:FindFirstChild(seat.Occupant.Parent.Name)
			ui:Clone().Parent = player.PlayerGui
			sound:Play()
		end
		player.PlayerGui:FindFirstChild("PoliceGUI"):Destroy()
		off:Play()
	end
end)