我看不到任何按钮或背景（Corona SDK）

我正在尝试为 Android Google Play 商店制作游戏。但是，在制作主菜单时遇到了问题。

在添加 storyboard 和 scene 函数之前，一切都运行正常。但是现在运行时什么也看不到。

另外，我没有收到任何错误。

    -- 需求
    local widget = require "widget"
    local storyboard = require ("storyboard")
    local scene = storyboard.newScene()

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

        -- 背景
        local background = display.newImage("images/bg.png")
        screenGroup:insert(background)

        -- 标题
        local title = display.newImage("images/title.png")
        title.x = display.contentCenterX
        title.y = display.contentCenterY - 110
        screenGroup:insert(title)

        -- 开始游戏
        local button1 = widget.newButton {
            label = "Play Game",
            font = default,
            fontSize = 24,
            width = 200,
            height = 50
        }
        button1.x = display.contentCenterX
        button1.y = display.contentCenterY - 47
        screenGroup:insert(button1)

        -- 游戏方法
        local button2 = widget.newButton {
            label = "How To Play",
            font = default,
            fontSize = 24,
            width = 200,
            height = 50
        }
        button2.x = display.contentCenterX
        button2.y = display.contentCenterY + 13
        screenGroup:insert(button2)

        -- 级别选择
        local button3 = widget.newButton {
            label = "Level Select",
            font = default,
            fontSize = 24,
            width = 200,
            height = 50
        }
        button3.x = display.contentCenterX
        button3.y = display.contentCenterY + 73
        screenGroup:insert(button3)

        -- 关于我们
        local button4 = widget.newButton {
            label = "About Us",
            font = default,
            fontSize = 24,
            width = 200,
            height = 50
        }
        button4.x = display.contentCenterX
        button4.y = display.contentCenterY + 133
        screenGroup:insert(button4)
    end

    function start(event)
        if event.phase == "began" then
            storyboard.gotoScene("level1", "fade", 400)
        end
    end

    function scene:enterScene(event)
        button1:addEventListener("touch", start)
    end

    return scene