addEventListener: listener cannot be nil:nil

Question

I'm creating a new game with Coronasdk, I need to make trees appear from the right of the screen and make them scroll to the left. That's the code

      local Trees = {}  --I generate trees that appears randomly
local function createTree(event)
table.insert(Trees, tree)
return tree
end

local function treeScroll (self, event)
  self.x = self.x - 3
end

MIN = 3000
MAX = 7000
local function gameLoop()
  local tree = createTree()
  tree.enterFrame = treeScroll
  Runtime:addEventListener("enterFrame", tree)
  for i, thisTree in ipairs (Trees) do
    if thisTree.x < -100 then
      Runtime:removeEventListener("enterFrame", thisTree)
      display.remove (thisTree)
      table.remove (Trees, i)
    end
  end
  timer.cancel(timer1)
  timer1=timer.performWithDelay(math.random(MIN, MAX), gameLoop, 0)
end

timer1 = timer.performWithDelay (math.random(MIN, MAX), gameLoop, 0)

It tells me that addEventListener:Listener cannot be nil:nil, and it refers to this part

local function gameLoop()
  local tree = createTree()
  tree.enterFrame = treeScroll
  Runtime:addEventListener("enterFrame", tree)
  for i, thisTree in ipairs (Trees) do
    if thisTree.x < -100 then
      Runtime:removeEventListener("enterFrame", thisTree)
      display.remove (thisTree)
      table.remove (Trees, i)
    end
  end
  timer.cancel(timer1)
  timer1=timer.performWithDelay(math.random(MIN, MAX), gameLoop, 0)
end

I don't know what to do