警报在Chrome扩展程序的弹出窗口中无效

Could any one help me,to find out my problem.I have set my website as chrome extension.When I install the extension,it navigates to a popup window asking user name and password along with a login button. But when I tried to alert the username entered by the user in javascript,it is not working.Anyone please help me.What would be the reason?Here is my manifest.jsonCould any one help me,to find out my problem.I