GWT code for ,same user login to the webpage again after loggedout from the page without refreshing the webpage?

 Hi All ! I have developed on website,in my website if one existed user is logged in to the webpage he can operate everything of that page,after he logged out from the page if he want to loggin again without refreshing the webpage it show NullPointeException,can you guys say the code for without refreshing the webpage can the same or any existed user can login successfully again once he loggedout from that in GWT? thanks ...bala

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