MainScene里用定时器时一直报attempt to call method 'Update' (a nil value) stack traceback:

我的cocos2dx版本是3.1.0、VS2013的，与老师的定时器写法一样的，也调试了多次，当运行截图的注释部分时蛇是可以移动的，就是用定时器就访问不了update了，这个怎么解决啊？困了我两天

MainScene.lua代码：

local MainScene = class("MainScene", cc.load("mvc").ViewBase)

local Snake = require "app.views.Snake";

--MainScene.RESOURCE_FILENAME = "MainScene.csb"

local cMoveSpeen = 0.3;

function MainScene:onEnter()

    self.snake = Snake.new(self)

    --self.snake:Update()

    --self.snake:Update()

    --self.snake:Update()

    --self.snake:Update()

    local tick = function()

       self.snake:Update()

    end;

    cc.Director:getInstance():getScheduler():scheduleScriptFunc(tick,cMoveSpeen,false);

end

return MainScene

Snake.lua代码：

local Snake = class("Snake",cc.load("mvc").ViewBase);

local Body = require "app.views.Body";

local cInitLen = 3;

function Snake:ctor(node)

    self.BodyArray = {}

    self.node = node

    self.MoveDir = "left"

    for i = 1,cInitLen  do

        self:Grow(i == 1)

    end

end

function Snake:GetTailGrid()

    if #self.BodyArray == 0 then

        return 0,0

    end

    local tail = self.BodyArray[#self.BodyArray];

    return tail.X,tail.Y

end

function Snake:Grow(isHead)

    local tailX,tailY = self:GetTailGrid();

    local body = Body.new(self,tailX,tailY,self.node,isHead);

    table.insert(self.BodyArray,body)

end

local function OffsetGridByDir(x,y,dir)

    if dir == "left" then

        return x-1,y

    elseif dir == "right" then

        return x+1,y

    elseif dir == "up" then

        return x,y-1

    elseif dir == "down" then

        return x,y+1

    end

    print("Unkown dir", dir)

    return x,y

end;

function Snake:Update()

   if #self.BodyArray == 0 then

        return 

    end

    for i = #self.BodyArray,1,-1 do

        local body = self.BodyArray[i];

        if i == 1 then

            body.X,body.Y = OffsetGridByDir(body.X,body.Y,self.MoveDir)

        else

            local front = self.BodyArray[i-1];

            body.X,body.Y = front.X,front.Y

        end

        body:Update()

    end

end

return Snake