in context with path [/POS] threw exception [com.vaadin.server.ServiceExcep

Dear all
I have main class that extends UI ‘MainUI’ i created a new vaadin project thats extends the MainUI whe i launch it i got that error

SEVERE: Servlet.service() for servlet [vc.pos.PosUI$Servlet]
in context with path
[/POS] threw exception [com.vaadin.server.ServiceException: com.vaadin.server.ServiceException: vc.pos.PosUI does not implement UI]
with root cause
com.vaadin.server.ServiceException: vc.pos.PosUI does not implement UI at com.vaadin.server.ServletPortletHelper.verifyUIClass(ServletPortletHelper.java:72)
i do compiled widgetset and resloved ivy
thanks in advance