最近在使用scheduler时发现如下问题

调用：

local scheduler = require(cc.PACKAGE_NAME .. ".scheduler")
 
 
function MainScene:ctor()
    
    self.player_:pos(self.player_:getContentSize().width / 2,display.height / 2)
   
    
    scheduler.scheduleGlobal(self.addMonster,1)
    
	
end
 
function MainScene:addMonster()
 
    local monster = display.newSprite("monster.png"):addTo(self)  
end

stack traceback:
	[string "framework/shortcodes.lua"]:62: in function 'addTo'
	[string "/Users/tokou/Documents/work_cocos/firstapp/src/app/scenes/MainScene.lua"]:30: in function <[string "/Users/tokou/Documents/work_cocos/firstapp/src/app/scenes/MainScene.lua"]:28>
cocos2d: ----------------------------------------

local scheduler = require(cc.PACKAGE_NAME .. ".scheduler")
 
 
function MainScene:ctor()
    
    self.player_:pos(self.player_:getContentSize().width / 2,display.height / 2)
   
    
   scheduler.scheduleGlobal(function()
        self:addMonster()
    end,1)
	
end
 
function MainScene:addMonster()
 
    local monster = display.newSprite("monster.png"):addTo(self)  
end