Listener not updating fields on transition screen

[Callum Carlile _Automation Consultants_]

I have a parent issue and a sub-task which both share a screen. I have created a custom listener which updates the fields when it is edited on the edit screen, but it does not work when it is updated on the transition screen. The listener listens to the 'Issue Updated' event, and I have configured the workflow transition post functions so that this event is fired here.

I read somewhere that this may be an event problem, but I am not sure what I am doing wrong. The code is as below:

import com.atlassian.jira.event.issue.IssueEvent
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.event.type.EventDispatchOption

def customFieldManager = ComponentAccessor.getCustomFieldManager()
def issueManager = ComponentAccessor.getIssueManager()
def changeHistoryManager= ComponentAccessor.getChangeHistoryManager()
def user = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()
def issue = event.getIssue()

if(issue.issueTypeObject.name == "Joiner Information" || issue.issueTypeObject.name == "Leaver Information"){
 //get the parent issue
 def parentIssue = issue.parentObject
 //get all the custom fields
 def customFields = customFieldManager.getCustomFieldObjects(issue)

 //get the changed item list from change log
 def changeLogs = event.getChangeLog()
 def changedItem = changeHistoryManager.getAllChangeItems(issue).findAll { it.changeGroupId == changeLogs.id}

 //mutating parent issue
 def mutableParentIssue = issueManager.getIssueObject(parentIssue.id)
 //update parent issue if the changed item is a custom field
 changedItem.each { item ->;
 if(customFields*.name.contains(item.field)) {
 def customFieldName = item.field
 def customField = customFieldManager.getCustomFieldObjectByName(customFieldName)
 def customFieldValue = issue.getCustomFieldValue(customField)
 mutableParentIssue.setCustomFieldValue(customField, customFieldValue)
 }
 }
 //updating the parent issue only if condition is true
 if(mutableParentIssue.getModifiedFields()){
 issueManager.updateIssue(user, mutableParentIssue, EventDispatchOption.ISSUE_UPDATED, false);
 }
}

I get the following error:

2019-02-04 13:18:28,739 ERROR [runner.AbstractScriptListener]: *************************************************************************************
2019-02-04 13:18:28,739 ERROR [runner.AbstractScriptListener]: Script function failed on event: com.atlassian.jira.event.issue.IssueEvent, file: <inline script>
java.lang.NullPointerException: Cannot get property 'id' on null object
	at Script2091.run(Script2091.groovy:22)
	at com.onresolve.scriptrunner.runner.ScriptRunnerImpl.runScriptAndGetContext(ScriptRunnerImpl.groovy:173)
	at com.onresolve.scriptrunner.runner.ScriptRunner$runScriptAndGetContext$2.callCurrent(Unknown Source)
	at com.onresolve.scriptrunner.runner.ScriptRunnerImpl.runStringAsScript(ScriptRunnerImpl.groovy:162)
	at com.onresolve.scriptrunner.runner.ScriptRunner$runStringAsScript$5.call(Unknown Source)
	at com.onresolve.scriptrunner.canned.jira.workflow.listeners.CustomListener.doScript(CustomListener.groovy:123)