错误：尝试对全局变量“cloud”进行索引。

为什么我看不到云图片，当我放置 x 和 y 坐标时，它说一个 nil 值，最后，我该如何让它随机向上运动并重置？

local storyboard = require("storyboard")
    local scene = storyboard.newScene()
    local physics = require "physics"
    physics.start( )
    physics.setGravity(0, 0)
    --physics.setDrawMode( "hybrid" )
    local LEFT = 100
    local CENTER = display.contentCenterX
    local RIGHT = display.contentWidth - 100

问题就在这里。我试图改变图片和各种编码，但失败了。

    function scene:createScene(event)
        local screenGroup = self.view

        cloud = display.newImage( "cloud.jpeg")
        cloud.x = 200
        cloud.y = 900

        back = display.newImage( "sky.png" )
        back.x = display.contentWidth/2
        back.y = display.contentHeight/2
        screenGroup:insert(back)

    end

   local function handleSwipe( event )
          if event.phase == "moved" then
               local dX = event.x - event.xStart
               print(event.x, event.xStart, dX)
               if dX > 10 then

                    local spot = RIGHT
                    if event.target.x == LEFT then
                        spot = CENTER
                   end
                   transition.to( event.target, {time=350, x = spot } )
              elseif dX  < -10 then

                   local spot = LEFT
                    if event.target.x == RIGHT then
                        spot = CENTER
                    end
                    transition.to( event.target, {time=350, x = spot } )
              end
           end
           return true
    end

    function scene:enterScene(event)

      object = display.newImage( "diver.png", display.contentCenterX,  display.contentHeight/1.7, 25 )

      object:addEventListener( "touch", handleSwipe )

     end

    function scene:exitScene( event )

    end

    function scene:destroyScene( event )
    end

    scene:addEventListener( "createScene", scene )
    scene:addEventListener( "enterScene", scene )
    scene:addEventListener( "exitScene", scene )
    scene:addEventListener( "destroyScene", scene )

    return scene

有人能帮忙吗？