ServerScriptService.PickUpServerScript:16: attempt to index nil with 'FindFirstChild'?

Making a pickup system for a game and keep getting this error in the output: 00:02:31.512 - ServerScriptService.PickUpServerScript:16: attempt to index nil with 'FindFirstChild'

here is the script:

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local PickupEvent = ReplicatedStorage:WaitForChild("PickupItem")

local SlotsFolder = game:GetService("ServerStorage"):FindFirstChild("ItemSlotsUI")
local SlotUIDictionary = {
	Water = SlotsFolder:FindFirstChild("Water")
}

local MagDist = 10
PickupEvent.OnServerEvent:connect(function(player, item)
	print(item)
	local Dist = player:DistanceFromCharacter(item.Handle.Position) 
	local playerGui = player:WaitForChild("PlayerGui")
	local MainFrame = playerGui:FindFirstChild("MainFrame")
	if Dist <= MagDist then
		if  MainFrame:FindFirstChild(item) then --ERROR IS HERE
			print('found')
		else
			print('not found')
		end
	end
end)

Give any suggestions, thanks :)