NPE in detach of AbstractClientConnector - Vaadin 7.7.15

I’ve a Form in wich, when I issue a detach, I get an NPE Exception.

The form is showed in a modal Popup.

Here the stacktrace.

Is it a bug ?

Tks
Tullio

com.vaadin.server.ServerRpcManager$RpcInvocationException: Unable to invoke method click in com.vaadin.shared.ui.button.ButtonServerRpc
	at com.vaadin.server.ServerRpcManager.applyInvocation(ServerRpcManager.java:162)
	at com.vaadin.server.ServerRpcManager.applyInvocation(ServerRpcManager.java:119)
	at com.vaadin.server.communication.ServerRpcHandler.handleInvocation(ServerRpcHandler.java:437)
	at com.vaadin.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:402)
	at com.vaadin.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:273)
	at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:90)
	at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
	at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1435)
	at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:380)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:800)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:800)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1471)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.vaadin.server.ServerRpcManager.applyInvocation(ServerRpcManager.java:158)
	... 31 more
Caused by: com.vaadin.event.ListenerMethod$MethodException: Invocation of method buttonClick in axioma.web.template.ui.bars.ToolbarItemBase failed.
	at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:533)
	at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:200)
	at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:163)
	at com.vaadin.server.AbstractClientConnector.fireEvent(AbstractClientConnector.java:1015)
	at com.vaadin.ui.Button.fireClick(Button.java:377)
	at com.vaadin.ui.Button$1.click(Button.java:54)
	... 36 more
Caused by: java.lang.NullPointerException
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:646)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:641)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:641)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:641)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:641)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:641)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:641)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:641)
	at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:701)
	at axioma.web.template.ui.frames.FreeFormFrame.handleFormEdit(FreeFormFrame.java:92)
	at axioma.web.template.ui.frames.DmsDocumentaleForm.handleFormEdit(DmsDocumentaleForm.java:68)
	at axioma.web.template.ui.frames.FreeFormFrame.handleEvent(FreeFormFrame.java:86)
	at axioma.web.template.ui.event.FrameEventManager.publishSingleEvent(FrameEventManager.java:37)
	at axioma.web.template.ui.event.FrameEventManager.publish(FrameEventManager.java:25)
	at axioma.web.template.ui.pages.MasterPage.publish(MasterPage.java:137)
	at axioma.web.template.ui.frames.InternalFrame.publish(InternalFrame.java:272)
	at axioma.web.template.ui.frames.ContainerFrame.goNextFrame(ContainerFrame.java:507)
	at axioma.web.template.ui.frames.ContainerFrame.goNextFrame(ContainerFrame.java:491)
	at axioma.web.template.ui.frames.ContainerFrame.insert(ContainerFrame.java:351)
	at axioma.web.template.ui.frames.ContainerFrame.executeCommand(ContainerFrame.java:461)
	at axioma.web.template.ui.frames.TableFrame.executeCommand(TableFrame.java:318)
	at axioma.web.template.ui.frames.DmsDocumentaleTable.executeCommand(DmsDocumentaleTable.java:101)
	at axioma.web.template.ui.commands.ToolbarCommand.execute(ToolbarCommand.java:48)
	at axioma.web.template.ui.bars.ToolbarItemBase.buttonClick(ToolbarItemBase.java:60)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:510)
	... 41 more

I got the same problem during attach.
getUI returns null.