java.lang.RuntimeException: Failed to serialize jenkins.model.Jenkins#securityRealm for class hudson.model.Hudson
	at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:256)
	at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:224)
	at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:138)
	at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:209)
	at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:150)
	at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
	at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
	at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43)
	at com.thoughtworks.xstream.core.TreeMarshaller.start(TreeMarshaller.java:82)
	at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.marshal(AbstractTreeMarshallingStrategy.java:37)
	at com.thoughtworks.xstream.XStream.marshal(XStream.java:1026)
	at com.thoughtworks.xstream.XStream.marshal(XStream.java:1015)
	at com.thoughtworks.xstream.XStream.toXML(XStream.java:988)
	at hudson.XmlFile.write(XmlFile.java:171)
	at jenkins.model.Jenkins.save(Jenkins.java:3180)
	at jenkins.model.Jenkins.saveQuietly(Jenkins.java:3186)
	at jenkins.model.Jenkins.setSecurityRealm(Jenkins.java:2569)
	at jenkins.model.Jenkins$setSecurityRealm$2.call(Unknown Source)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
	at github-realm.run(github-realm.groovy:22)
	at groovy.lang.GroovyShell.evaluate(GroovyShell.java:585)
	at jenkins.util.groovy.GroovyHookScript.execute(GroovyHookScript.java:136)
	at jenkins.util.groovy.GroovyHookScript.execute(GroovyHookScript.java:127)
	at jenkins.util.groovy.GroovyHookScript.run(GroovyHookScript.java:110)
	at hudson.init.impl.GroovyInitScript.init(GroovyInitScript.java:41)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at hudson.init.TaskMethodFinder.invoke(TaskMethodFinder.java:104)
	at hudson.init.TaskMethodFinder$TaskImpl.run(TaskMethodFinder.java:175)
	at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
	at jenkins.model.Jenkins$7.runTask(Jenkins.java:1088)
	at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
	at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
	at com.thoughtworks.xstream.io.xml.PrettyPrintWriter.writeText(PrettyPrintWriter.java:235)
	at com.thoughtworks.xstream.io.xml.PrettyPrintWriter.writeText(PrettyPrintWriter.java:231)
	at com.thoughtworks.xstream.io.xml.PrettyPrintWriter.setValue(PrettyPrintWriter.java:214)
	at com.thoughtworks.xstream.io.WriterWrapper.setValue(WriterWrapper.java:45)
	at org.jenkinsci.plugins.GithubSecurityRealm$ConverterImpl.marshal(GithubSecurityRealm.java:236)
	at hudson.util.XStream2$AssociatedConverterImpl.marshal(XStream2.java:370)
	at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
	at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
	at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:84)
	at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:265)
	at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:252)
	... 39 more

import jenkins.model.*

import hudson.security.SecurityRealm
import org.jenkinsci.plugins.GithubSecurityRealm


String githubWebUri = 'https://github.com'
String githubApiUri = 'https://api.github.com'
String oauthScopes = 'read:org,user:email'
String clientSecret = 'secret'
String clientID = 'clientid'

  SecurityRealm github_realm = new GithubSecurityRealm(githubWebUri, githubApiUri, clientID, clientSecret, oauthScopes)

  //check for equality, no need to modify the runtime if no settings changed
  if(!github_realm.equals(Jenkins.instance.getSecurityRealm()) ) {
    Jenkins.instance.setSecurityRealm(github_realm)
    Jenkins.instance.save()
  }