尝试索引全局控件 (一个 nil 值)。

每次运行此代码，都会显示错误：

尝试索引全局变量 widget（一个“nil”值）

local inc = 0.0
local levelUpBar, player
levelUpBar = widget.newProgressView {
   left = display.contentWidth*0.5-100,
   top = 300,
   width = 200,
   isAnimated = true }
levelUpBar:setProgress( 0 )

local function playerHit(event)
    if(event.phase == "began") then
        inc = inc + 0.2
        if(inc > 1) then
            print('level up!')
        else
            levelUpBar:setProgress( inc )
        end
    end
end

player = display.newCircle(0,0,75)
player.x = display.contentWidth*0.5
player.y = 200
player:setFillColor(0,200,0)
player:addEventListener("touch", playerHit)