Cannot set property 'innerHTML' of null

mainfest.json

"content_scripts": [
		{
		  "js": ["js/content_script.js"],
		  "matches": [
			"http://mp.weixin.qq.com/*",
			"https://mp.weixin.qq.com/*"
		  ],
		  "run_at": "document_start"
		}
	  ],

解決辦法

"content_scripts": [
		{
		  "js": ["js/content_script.js"],
		  "matches": [
			"http://mp.weixin.qq.com/*",
			"https://mp.weixin.qq.com/*"
		  ],
		  "run_at": "document_end"
		}
	  ],