1. Created job with "None" SCM, saved successfully
2. Configure the job, set SCM to "Filesystem"
3. Click "Save".

Hudson shows "Error 500" with the following stack trace.

java.lang.RuntimeException: Failed to serialize hudson.model.AbstractProject#scm for class hudson.model.FreeStyleProject
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:167)
at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:135)
at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:130)
at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:120)
at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:94)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:68)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:78)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:63)
at com.thoughtworks.xstream.core.TreeMarshaller.start(TreeMarshaller.java:98)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.marshal(AbstractTreeMarshallingStrategy.java:38)
at com.thoughtworks.xstream.XStream.marshal(XStream.java:840)
at com.thoughtworks.xstream.XStream.marshal(XStream.java:829)
at com.thoughtworks.xstream.XStream.toXML(XStream.java:804)
at hudson.XmlFile.write(XmlFile.java:165)
at hudson.model.AbstractItem.save(AbstractItem.java:225)
at hudson.model.Job.save(Job.java:161)
at hudson.model.AbstractProject.setScm(AbstractProject.java:1223)
at hudson.model.AbstractProject.submit(AbstractProject.java:1523)
at hudson.model.Project.submit(Project.java:191)
at hudson.model.FreeStyleProject.submit(FreeStyleProject.java:97)
at hudson.model.Job.doConfigSubmit(Job.java:1062)
at hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:578)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:259)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:126)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:65)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:79)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:534)
at org.kohsuke.stapler.MetaClass$7.doDispatch(MetaClass.java:219)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:534)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:450)
at org.kohsuke.stapler.Stapler.service(Stapler.java:132)
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.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
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:636)
Caused by: java.lang.RuntimeException: Failed to serialize hudson.plugins.filesystem_scm.FSSCM#filters for class hudson.plugins.filesystem_scm.FSSCM
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:167)
at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:135)
at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:130)
at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:120)
at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:94)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:68)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:78)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:63)
at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:175)
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:163)
... 56 more
Caused by: java.lang.NullPointerException
at java.lang.Class.isAssignableFrom(Native Method)
at hudson.util.XStream2$1.serializedClass(XStream2.java:115)
at com.thoughtworks.xstream.mapper.MapperWrapper.serializedClass(MapperWrapper.java:34)
at com.thoughtworks.xstream.mapper.MapperWrapper.serializedClass(MapperWrapper.java:34)
at com.thoughtworks.xstream.converters.collections.AbstractCollectionConverter.writeItem(AbstractCollectionConverter.java:58)
at com.thoughtworks.xstream.converters.collections.ArrayConverter.marshal(ArrayConverter.java:45)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:68)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:78)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:63)
at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:175)
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:163)
... 65 more