Corona SDK主菜单的Ui按钮

我在使用 Corona SDK 中有点新手，在我的主菜单按钮上遇到了一点问题。每当我按下按钮时，它不会移动或更改视图；按钮只会在标题屏幕中消失。

module(..., package.seeall)

local ui = require ("ui")
local ui = require ("director")

local assetPath = "assets/"

local mainGroup = display.newGroup()

function new(params)
    local titleScreen = display.newImageRect(assetPath .. "Law of Magic.jpg",
        display.contentWidth, display.contentHeight)
    titleScreen.x = display.contentWidth / 2
    titleScreen.y = 265
    mainGroup:insert(titleScreen)
    director:changeScene("titleScreen")

    local newgame = ui.newButton{ default = assetPath .. "new game.png",
        onRelease = function(event)  director:changeScene("New game") end,}
    newgame.x = display.contentWidth / 2
    newgame.y = 445
    mainGroup:insert(newgame)

    local continue = ui.newButton{ default = assetPath .. "continue.png",
        onRelease = function(event)  director:changeScene("Continue") end,}
    continue.x = display.contentWidth / 2
    continue.y = 447
    mainGroup:insert(continue)

    local option = ui.newButton{ default = assetPath .. "option.png",
        onRelease = function(event)  director:changeScene("Option") end,}
    option.x = display.contentWidth / 2
    option.y = 449
    mainGroup:insert(option)

    local help = ui.newButton{ default = assetPath .. "help.png",
        onRelease = function(event)  director:changeScene("Help") end,}
    help.x = display.contentWidth / 2
    help.y = 451
    mainGroup:insert(help)

    local exitgame = ui.newButton{ default = assetPath .. "exit game.png",
        onRelease = function(event)  director:changeScene("Exit game") end,}
    exitgame.x = display.contentWidth / 2
    exitgame.y = 453
    mainGroup:insert(exitgame)

    return mainGroup
end