What is wrong with my Spray Bottle Script?

Basically, the gear script eliminates every decal one-by-one (I know how to assert, but I see potential problems doing so).

But, now and then, I get a "LocalScript:65: attempt to index field '?' (a nil value)" error while using the gear, and I was wondering if anyone could help me fix this. Thanks in advance.

sp = script.Parent
spraypart = script.Parent.SprayPart


cooldown = 2.1
spraysize = 5
range = 10

local debris = game:GetService("Debris")
equipped = false
check = true

function onEquipped(mouse)
	if spraypart ~= nil and spraypart.Parent ~= nil then
		local weld = Instance.new("Weld")
		weld.Name = "SprayerWeld"
		weld.Part0 = spraypart
		weld.Part1 = sp.Handle
		
		-- correction term to account for average skew between physics update and heartbeat
		local HitPos = spraypart.Position --+ (-arrow.Velocity * (1/60)) --+ (arrow.CFrame.lookVector * .5)
		
		local CJ = CFrame.new(HitPos)
		local C0 = spraypart.CFrame:inverse() * CJ
		local C1 = sp.Handle.CFrame:inverse() * CJ
		
		weld.C0 = C0
		weld.C1 = C1
		
		weld.Parent = sp.Handle
	end
	equipped = true
	local plr = game.Players.LocalPlayer
	if mouse ~= nil and plr ~= nil then
		mouse.Button1Down:connect(function()
			
			local chr = sp.Parent
			if chr and check and mouse.Target ~= nil and mouse.TargetSurface ~= nil and sprayid > 0 then
				local surface = mouse.TargetSurface
				local t = chr:FindFirstChild("Torso")
				local h = chr:FindFirstChild("Humanoid")
				local animobject = sp:FindFirstChild("SprayPaint")
				if t and h and animobject then
					if h.Health > 0 then
						check = false
						local anim = h:LoadAnimation(animobject)
						if anim then
							anim:Play()
							sp.Handle.Sound:Play()
						end
						if spraypart ~= nil and spraypart.Parent ~= nil then
							spraypart.SprayEffect.Enabled = true
						end
						wait()
						for i = 1, #mouse.Target:GetChildren() do
							if mouse.Target:GetChildren()[i]:IsA('Decal') or mouse.Target:GetChildren()[i]:IsA('Texture') then
								repeat
									mouse.Target:GetChildren()[i].Transparency = mouse.Target:GetChildren()[i].Transparency + .1
									wait(.1)
								until mouse.Target:GetChildren()[i].Transparency > 1
								mouse.Target:GetChildren()[i].Transparency = 1
								mouse.Target:GetChildren()[i].Texture = 'http://www.roblox.com/images/empty.png'
							end
						end
					end
					wait(cooldown)
					if spraypart ~= nil and spraypart.Parent ~= nil then
						spraypart.SprayEffect.Enabled = false
					end
					sp.Handle.Sound:Stop()
					check = true
				end
			end
		end)
	end
end

function onUnequipped()

	equipped = false
end

sp.Equipped:connect(onEquipped)
sp.Unequipped:connect(onUnequipped)