Why does it randomly say "attempt to index a nil value?"

I am trying to use a script where a billboard gui pops up on the closest part to the player that the player spawned using a button. I tried using this script:

local plrName = script.Parent.Name
local rK = game.Players:GetPlayerFromCharacter(script.Parent).PlayerGui:WaitForChild("RKeyGen")
n = nil

while true do
    for i, v in pairs(game.Workspace.Bricks:GetChildren())do
        if v.Name == plrName then
            local pPos = v.Position
            local lPos = script.Parent:FindFirstChild("HumanoidRootPart").Position
            local sAway = (pPos - lPos).magnitude
            if sAway ~= nil then
                v:FindFirstChild("NumVal").Value = tonumber(sAway)
                print(sAway)
                if n ~= nil then
                    if v:FindFirstChild("NumVal").Value > n then
                        rK.Enabled = true
                        rK.Adornee = v
                    else
                        n = v:FindFirstChild("NumVal").Value
                    end
                else
                    n = v:FindFirstChild("NumVal").Value
                end
            end
        end
    end
    wait(0.15)
end

It works sometimes, but then an error occurs randomly and says: "Workspace.snaks308070.Script:12: attempt to index a nil value"

Any help?