我有Vaadin 7申请。当我打开一些弹出窗口并让应用程序空闲30-40分钟时，关闭此窗口时我得到了NPE。这里有相关的痕迹：
java.lang.NullPointerException: null
at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:603) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:598) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:589) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:598) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:589) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:598) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:589) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:598) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:589) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.AbstractComponent.setParent(AbstractComponent.java:471) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.Window.setParent(Window.java:153) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.UI.removeWindow(UI.java:514) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.Window.close(Window.java:244) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.ui.Window.changeVariables(Window.java:206) ~[vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.communication.ServerRpcHandler.changeVariables(ServerRpcHandler.java:396) [vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.communication.ServerRpcHandler.handleBurst(ServerRpcHandler.java:221) [vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:111) [vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:91) [vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:37) [vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1382) [vaadin-server-7.1.10.jar:7.1.10]
at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:238) [vaadin-server-7.1.10.jar:7.1.10]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727) [servlet-api.jar:na]
可以看出，AbstractClientConnector.detach触发两次，我在getUI（）方法调用上获得了NPE。可能是什么问题？
由于