java.io.NotSerializableException: java.util.ArrayList$Itr HashMap Jenkins Pipeline

Question

I have a groovy functions that converts a local variable into a map and allocates the variable to an environmental variable.

def variableMap() {
  def var = varList.toSpreadMap()
  env.varMap = "${var}"
}

I use the environmental variable in the pipeline stages and iterate over it.

@Library('my_library') _
pipeline {
 stages {
  mylibrary.variableMap()

  stage ('One') {
  steps {
   env.varMap.each { key,value ->
                 echo "${key}"
                 echo "${value}"
                 ...
    }
  }
 }
}

However, for some reason I get the following error:

Caused: java.io.NotSerializableException: java.util.ArrayList$Itr
    at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:926)
    at org.jboss.marshalling.river.BlockMarshaller.doWriteObject(BlockMarshaller.java:65)
    at org.jboss.marshalling.river.BlockMarshaller.writeObject(BlockMarshaller.java:56)
    at org.jboss.marshalling.MarshallerObjectOutputStream.writeObjectOverride(MarshallerObjectOutputStream.java:50)
    at org.jboss.marshalling.river.RiverObjectOutputStream.writeObjectOverride(RiverObjectOutputStream.java:179)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:344)
    at java.util.HashMap.internalWriteEntries(HashMap.java:1793)
    at java.util.HashMap.writeObject(HashMap.java:1363)
    at sun.reflect.GeneratedMethodAccessor481.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.jboss.marshalling.reflect.JDKSpecific$SerMethods.callWriteObject(JDKSpecific.java:156)
    at org.jboss.marshalling.reflect.SerializableClass.callWriteObject(SerializableClass.java:191)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1028)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:920)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteFields(RiverMarshaller.java:1082)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1040)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:920)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteFields(RiverMarshaller.java:1082)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1040)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1019)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:920)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteFields(RiverMarshaller.java:1082)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1040)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1019)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1019)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:920)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteFields(RiverMarshaller.java:1082)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1040)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:1019)
    at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:920)

What is the reason for this issue? Is there a way to solve this?

Note: I found a similar post. But I am not sure how the workaround can help me in this case.

How to fix NotSerializableException error during Jenkins workflow build?

Also I cannot understand the explanation behind the issue.

Edit

Also worth mentioning that I annotated the method as some solutions online suggest. But the result was the same:

@NonCPS
def variableMap() {
  def var = varList.toSpreadMap()
  env.varMap = "${var}"
}