Vaadin 7 + Spring boot 1.0.2 session problems

I using vaadin + spring boot, and I getting some problem with the session, If my user access my app, (he have to make one login, in this login I save the user in a session var

((CartorioUI) UI.getCurrent()).setUsuario(usuario);

and the spring create a navigator,

but when my user log in my app, and refresh the page(back to login form) sometimes after login he don`t save the user, and I got the UI has no navigator error, then I need to clear all browser cache to join again… anyone know how fix this? or make the vaadin force to use a new session for every time that my user try to access (even if he just reload the page)?

tks