NullPointerException in AbstractClientConnector - help or pointer to code?

I posted my problem in the "Miscellaneous" forum but decided to post here also, since Navigator is really a framework component.  Plus I can't seem to find the exact version of the source code to try and track down the problem, so I could really use some help.   First the scenario:
I created a very simple application with two Views.  The UI.init() creates a Navigator and registers two views via calls:

    navigator.addView("", new LoginView());
    navigator.addView("MainView", new MainView());

In the LoginView's Button.ClickListener()'s buttonClick() method, I call
    navigator.navigateTo("MainView");
When that call is made, the NullPointerException below happens.  I went to github and cloned the vaadin repository and tried to find the source code in the stack trace.  But github doesn't have a 7.5.1 branch and when I switch to the 7.5 branch, the line numbers given in the stack trace don't line up with the sourcce code.

Environment: I'm using the Vaadin 7.5.1 with Java 8u51 and Tomcat 8.0.20 on a Windows 7 box.

Any help/pointers are really appreciated.  I'm just getting started on this project and to get stuck so soon (the View I'm navigating to just has a single button - just to see something) is depressing.  Especially since at a previous company/project, when I used Vaadin (7.2 at the time), I had no trouble whatsoever with navigation between Views.

Thanks in advance,
tom

----- NullPointerException -------
java.lang.NullPointerException
    at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:628)
    at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:686)
    at com.vaadin.server.AbstractClientConnector.detach(AbstractClientConnector.java:623)
    at com.vaadin.ui.AbstractComponent.detach(AbstractComponent.java:686)
    at com.vaadin.ui.AbstractComponent.setParent(AbstractComponent.java:568)
    at com.vaadin.ui.AbstractSingleComponentContainer.setContent(AbstractSingleComponentContainer.java:150)
    at com.vaadin.ui.UI.setContent(UI.java:1305)
    at com.vaadin.navigator.Navigator$SingleComponentContainerViewDisplay.showView(Navigator.java:224)
    at com.vaadin.navigator.Navigator.navigateTo(Navigator.java:575)
    at com.vaadin.navigator.Navigator.navigateTo(Navigator.java:527)
    at com.lenovo.sat.satweb.LoginView$1.buttonClick(LoginView.java:40)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.vaadin.event.ListenerMethod.receiveEvent(ListenerMethod.java:508)
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:198)
    at com.vaadin.event.EventRouter.fireEvent(EventRouter.java:161)
    at com.vaadin.server.AbstractClientConnector.fireEvent(AbstractClientConnector.java:995)
    at com.vaadin.ui.Button.fireClick(Button.java:380)
    at com.vaadin.ui.Button.click(Button.java:369)
    at com.vaadin.ui.Button$ClickShortcut.handleAction(Button.java:548)
    at com.vaadin.event.ActionManager.handleAction(ActionManager.java:238)
    at com.vaadin.event.ConnectorActionManager.handleAction(ConnectorActionManager.java:81)
    at com.vaadin.event.ActionManager.handleAction(ActionManager.java:233)
    at com.vaadin.event.ActionManager.handleActions(ActionManager.java:216)
    at com.vaadin.ui.UI.changeVariables(UI.java:390)
    at com.vaadin.server.communication.ServerRpcHandler.changeVariables(ServerRpcHandler.java:508)
    at com.vaadin.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:327)
    at com.vaadin.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:202)
    at com.vaadin.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:95)
    at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
    at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1408)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:350)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:537)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1085)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:658)
    at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1556)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1513)
    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)