If the slave is not on line and the user tries to access the workspace through the gui
a NPE is generated.

Steps to reproduce
1. Create a slave
2. Start slave through web start
3. Create job that is tied to the slave
4. Build job to generate the workspace
5. Verify that the workspace can be accessed
6. Kill the web start app
7. Try to access the workspace and the NPE is shown

Exception:
java.lang.NullPointerException
at hudson.model.AbstractProject.doWs(AbstractProject.java:927)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:103)
at org.kohsuke.stapler.Function.bindAndinvoke(Function.java:57)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:69)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:416)
at org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:248)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:416)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:352)
at org.kohsuke.stapler.Stapler.service(Stapler.java:112)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:45)
at winstone.ServletConfiguration.execute(ServletConfiguration.java:249)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:335)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:378)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:88)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:333)
at winstone.RequestHandlerThread.processRequest(RequestHandlerThread.java:244)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:150)
at java.lang.Thread.run(Thread.java:619)