What does this error mean? " The Parent property is locked, current parent: NULL

This code works until the character dies and then respawns. If they click on the item once they have respawned I get this error " The Parent property of Science is locked, current parent: NULL, new parent Backpack" And it will not clone the item into the players backpack. The only thing I could find in Wiki was that a parent property locked and set to null on an instance happens when an object is removed or destroyed, which would obviously happen when the player dies, so why would that error occur once a character respawns and how can I fix that?

local RE = game.ReplicatedStorage
local event = RE.Notify
local event2 = RE.ScienceEvent
local klone = game.ServerStorage:FindFirstChild("Science"):Clone()
db = false
event2.OnServerEvent:Connect(function(plr)
	if db == false then
	db = true
			if klone.Parent ~= plr.Backpack then
				klone.Parent = plr.Backpack
			    event:FireAllClients(plr.Name.." found the Science Room Key!")
			end
	wait(35)
	db = false
	end
end)

--Here is the local script

local clickdetector = game.Workspace.Science.h.ClickDetector
clickdetector.MouseClick:Connect(function()
	game.ReplicatedStorage.ScienceEvent:FireServer()
end)