What's wrong with this script?

When a tool is equipped, the Gui clones and the player can see it. That works fine. The problem is that when it's unequipped, it doesn't destroy the Gui. On the output it says something like "attempted to index guiMain, a nil value". Anyone know how to fix this?

Tool = script.Parent
guiMain = nil

function onEquippedLocal()
    local character = script.Parent.Parent
    local player = game.Players:GetPlayerFromCharacter(character)
    local guiMain = script.Parent.AirportControl:clone()
    guiMain.Parent = player.PlayerGui
end

function onUnequippedLocal()
    guiMain:Destroy()
end

Tool.Equipped:connect(onEquippedLocal)
Tool.Unequipped:connect(onUnequippedLocal)