按照官网的代码抄的，也会报错 这个怎么回事
CCBoot.js:690 Uncaught TypeError: Cannot read property ‘toLowerCase’ of null
_loadResIterator @ CCBoot.js:690
(anonymous function) @ CCBoot.js:768
self._handleItem @ CCBoot.js:157
self.flow @ CCBoot.js:189
load @ CCBoot.js:777
_startLoading @ CCLoaderScene.js:116
_doCallback @ CCScheduler.js:154
update @ CCScheduler.js:172
update @ CCScheduler.js:370
drawScene @ CCDirector.js:220
mainLoop @ CCDirector.js:869
callback @ CCBoot.js:1801