got this while saving System Configuration in Jenkins.

I have this plugin installed: "Openstack Cloud Plugin
Allows Jenkins to build using Cloud Servers via JClouds." - Version 1.7.

this may be applicable to jclouds-plugin as well (see my fix for both below)

my "retention time" is set to default: 30 (minutes I assume)
and on "Cloud Instance Templates" level the value shown for retention-override is shown as "0" in the web UI.

javax.servlet.ServletException: java.lang.NullPointerException
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:796)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:686)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1494)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:123)
at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter$1.call(ScmSyncConfigurationFilter.java:46)
at hudson.plugins.scm_sync_configuration.ScmSyncConfigurationDataProvider.provideRequestDuring(ScmSyncConfigurationDataProvider.java:103)
at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter.doFilter(ScmSyncConfigurationFilter.java:42)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:120)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:114)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:48)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1482)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1474)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:499)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:533)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:428)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
at org.eclipse.jetty.server.Server.handle(Server.java:370)
at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:489)
at org.eclipse.jetty.server.AbstractHttpConnection.content(AbstractHttpConnection.java:960)
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.content(AbstractHttpConnection.java:1021)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:865)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:240)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:668)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
at jenkins.plugins.openstack.compute.JCloudsSlave.getRetentionTime(JCloudsSlave.java:109)
at jenkins.plugins.openstack.compute.JCloudsComputer.getRetentionTime(JCloudsComputer.java:37)
at jenkins.plugins.openstack.compute.JCloudsRetentionStrategy.check(JCloudsRetentionStrategy.java:32)
at jenkins.plugins.openstack.compute.JCloudsRetentionStrategy.check(JCloudsRetentionStrategy.java:16)
at hudson.slaves.SlaveComputer$4.run(SlaveComputer.java:707)
at hudson.model.Queue._withLock(Queue.java:1286)
at hudson.model.Queue.withLock(Queue.java:1169)
at hudson.slaves.SlaveComputer.setNode(SlaveComputer.java:704)
at hudson.model.AbstractCIBase.updateComputer(AbstractCIBase.java:118)
at hudson.model.AbstractCIBase.access$000(AbstractCIBase.java:44)
at hudson.model.AbstractCIBase$2.run(AbstractCIBase.java:186)
at hudson.model.Queue._withLock(Queue.java:1286)
at hudson.model.Queue.withLock(Queue.java:1169)
at hudson.model.AbstractCIBase.updateComputerList(AbstractCIBase.java:169)
at jenkins.model.Jenkins.updateComputerList(Jenkins.java:1224)
at jenkins.model.Jenkins.doConfigSubmit(Jenkins.java:2881)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:298)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:161)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:96)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:121)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
... 63 more

now I see that Openstack jenkins workers are not being deleted when they should be...
log is full of these messages:

Jun 30, 2015 10:35:46 AM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.slaves.ComputerRetentionWork@181dfa27 failed
java.lang.NullPointerException
at jenkins.plugins.openstack.compute.JCloudsSlave.getRetentionTime(JCloudsSlave.java:109)
at jenkins.plugins.openstack.compute.JCloudsComputer.getRetentionTime(JCloudsComputer.java:37)
at jenkins.plugins.openstack.compute.JCloudsRetentionStrategy.check(JCloudsRetentionStrategy.java:32)
at jenkins.plugins.openstack.compute.JCloudsRetentionStrategy.check(JCloudsRetentionStrategy.java:16)
at hudson.slaves.ComputerRetentionWork$1.run(ComputerRetentionWork.java:70)
at hudson.model.Queue._withLock(Queue.java:1286)
at hudson.model.Queue.withLock(Queue.java:1169)
at hudson.slaves.ComputerRetentionWork.doRun(ComputerRetentionWork.java:61)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:51)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:178)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)