java.lang.RuntimeException: Failed to serialize hudson.plugins.xvnc.Xvnc$DescriptorImpl#allocators for class hudson.plugins.xvnc.Xvnc$DescriptorImpl
	at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:215)
	at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:183)
	at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:138)
	at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:168)
	at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:109)
	at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
	at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
	at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43)
	at com.thoughtworks.xstream.core.TreeMarshaller.start(TreeMarshaller.java:82)
	at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.marshal(AbstractTreeMarshallingStrategy.java:37)
	at com.thoughtworks.xstream.XStream.marshal(XStream.java:1026)
	at com.thoughtworks.xstream.XStream.marshal(XStream.java:1015)
	at com.thoughtworks.xstream.XStream.toXML(XStream.java:988)
	at hudson.XmlFile.write(XmlFile.java:178)
	at hudson.model.Descriptor.save(Descriptor.java:855)
	at hudson.plugins.xvnc.DisplayAllocator.save(DisplayAllocator.java:32)
	at hudson.plugins.xvnc.DisplayAllocator.allocate(DisplayAllocator.java:51)
	at hudson.plugins.xvnc.Xvnc.doSetUp(Xvnc.java:104)
	at hudson.plugins.xvnc.Xvnc.setUp(Xvnc.java:94)
	at org.jenkinsci.plugins.workflow.steps.CoreWrapperStep$Execution.start(CoreWrapperStep.java:74)
	at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:138)
	at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:113)
	at groovy.lang.GroovyObject$invokeMethod$10.call(Unknown Source)
	at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:42)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:108)
	at org.kohsuke.groovy.sandbox.impl.Checker$1.call(Checker.java:151)
	at org.kohsuke.groovy.sandbox.GroovyInterceptor.onMethodCall(GroovyInterceptor.java:21)
	[....]
Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1463)
	at java.util.HashMap$EntryIterator.next(HashMap.java:1461)
	at com.thoughtworks.xstream.converters.collections.MapConverter.marshal(MapConverter.java:75)
	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:224)
	at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:211)
	at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:183)
	at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:138)
	at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:168)
	at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:109)
	at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
	at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
	at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43)
	at com.thoughtworks.xstream.core.TreeMarshaller.start(TreeMarshaller.java:82)
	at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.marshal(AbstractTreeMarshallingStrategy.java:37)
	at com.thoughtworks.xstream.XStream.marshal(XStream.java:1026)
	at com.thoughtworks.xstream.XStream.marshal(XStream.java:1015)
	at com.thoughtworks.xstream.XStream.toXML(XStream.java:988)
	at hudson.XmlFile.write(XmlFile.java:178)
	at hudson.model.Descriptor.save(Descriptor.java:855)
	at hudson.plugins.xvnc.DisplayAllocator.save(DisplayAllocator.java:32)
	at hudson.plugins.xvnc.DisplayAllocator.allocate(DisplayAllocator.java:51)
	at hudson.plugins.xvnc.Xvnc.doSetUp(Xvnc.java:104)
	at hudson.plugins.xvnc.Xvnc.setUp(Xvnc.java:94)
	at org.jenkinsci.plugins.workflow.steps.CoreWrapperStep$Execution.start(CoreWrapperStep.java:74)
        [...]