If agents are not stopped directly but are retained for a while, then restarting Jenkins will make those agents unusable. When running a job after a restart I see thisin the  console:

 
Started by user
Lars Berntzon
Obtained tmp/xxxx/Jenkinsfile from git ssh://gerrit-server:29418/git-repo
Running in Durability level: MAX_SURVIVABILITY[Pipeline] Start of Pipeline[Pipeline] nodeRunning on agent-b4nm0 in /home/jenkins/workspace/project[Pipeline]

[Pipeline] // node[Pipeline] End of Pipelinejava.lang.IllegalStateException: Not expecting pod template to be null at this point
at org.csanchez.jenkins.plugins.kubernetes.KubernetesSlave.getTemplate(KubernetesSlave.java:92)
at org.csanchez.jenkins.plugins.kubernetes.pipeline.SecretsMasker$Factory.secretsOf(SecretsMasker.java:144)
at org.csanchez.jenkins.plugins.kubernetes.pipeline.SecretsMasker$Factory.get(SecretsMasker.java:122)
at org.csanchez.jenkins.plugins.kubernetes.pipeline.SecretsMasker$Factory.get(SecretsMasker.java:94)
at org.jenkinsci.plugins.workflow.steps.DynamicContext$Typed.get(DynamicContext.java:94)
at org.jenkinsci.plugins.workflow.cps.ContextVariableSet.get(ContextVariableSet.java:139)
at org.jenkinsci.plugins.workflow.cps.CpsThread.getContextVariable(CpsThread.java:135)
at org.jenkinsci.plugins.workflow.cps.CpsStepContext.doGet(CpsStepContext.java:297)
at org.jenkinsci.plugins.workflow.support.DefaultStepContext.get(DefaultStepContext.java:75)
at org.jenkinsci.plugins.workflow.support.DefaultStepContext.getListener(DefaultStepContext.java:127)
at org.jenkinsci.plugins.workflow.support.DefaultStepContext.get(DefaultStepContext.java:79)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeStep(DSL.java:258)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:193)
at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:122)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:48)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.methodCall(DefaultInvoker.java:20)
at org.jenkinsci.plugins.pipeline.modeldefinition.agent.CheckoutScript.performCheckout(CheckoutScript.groovy:72)
at org.jenkinsci.plugins.pipeline.modeldefinition.agent.CheckoutScript.checkoutAndRun(CheckoutScript.groovy:56)
at org.jenkinsci.plugins.pipeline.modeldefinition.agent.CheckoutScript.doCheckout(CheckoutScript.groovy:40)
at org.jenkinsci.plugins.pipeline.modeldefinition.agent.impl.LabelScript.run(LabelScript.groovy:43)
at __cps.transform__(Native Method)
at com.cloudbees.groovy.cps.impl.ContinuationGroup.methodCall(ContinuationGroup.java:86)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.dispatchOrArg(FunctionCallBlock.java:113)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.fixArg(FunctionCallBlock.java:83)
at jdk.internal.reflect.GeneratedMethodAccessor218.invoke(Unknown Source)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.ClosureBlock.eval(ClosureBlock.java:46)
at com.cloudbees.groovy.cps.Next.step(Next.java:83)
at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:174)
at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:163)
at org.codehaus.groovy.runtime.GroovyCategorySupport$ThreadCategoryInfo.use(GroovyCategorySupport.java:129)
at org.codehaus.groovy.runtime.GroovyCategorySupport.use(GroovyCategorySupport.java:268)
at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:163)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.access$001(SandboxContinuable.java:18)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.run0(SandboxContinuable.java:51)
at org.jenkinsci.plugins.workflow.cps.CpsThread.runNextChunk(CpsThread.java:185)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.run(CpsThreadGroup.java:400)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.access$400(CpsThreadGroup.java:96)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:312)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:276)
at org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService$2.call(CpsVmExecutorService.java:67)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:139)
at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at jenkins.security.ImpersonatingExecutorService$1.run(ImpersonatingExecutorService.java:68)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
Finished: FAILURE