How to create a subtask using script runner listener (groovy script)?

We are working on using a script listener to automatically generate sub-tasks based on the values in a custom field. I've tried the solution based on this: https://community.atlassian.com/t5/Jira-questions/How-to-Automatically-create-Sub-task-using-script-runner-post/qaq-p/400564 but it's throwing the following error:

2018-01-08 03:20:27,207 ERROR [runner.AbstractScriptListener]: Script function failed on event: com.atlassian.jira.event.issue.IssueEvent, file: <inline script>
com.atlassian.jira.exception.CreateException
	at com.atlassian.jira.issue.managers.DefaultIssueManager.createIssue(DefaultIssueManager.java:588)
	at com.atlassian.jira.issue.managers.DefaultIssueManager.createIssue(DefaultIssueManager.java:494)
	at com.atlassian.jira.issue.managers.DefaultIssueManager.createIssueObject(DefaultIssueManager.java:599)
	at com.atlassian.jira.issue.managers.RequestCachingIssueManager.createIssueObject(RequestCachingIssueManager.java:198)
	at com.atlassian.jira.issue.IssueManager$createIssueObject.call(Unknown Source)
	at Script33.run(Script33.groovy:69)
Caused by: com.atlassian.jira.workflow.WorkflowException
	at com.atlassian.jira.workflow.OSWorkflowManager.createIssue(OSWorkflowManager.java:768)
	at com.atlassian.jira.issue.managers.DefaultIssueManager.createIssue(DefaultIssueManager.java:580)
	... 5 more
Caused by: java.lang.NullPointerException
	at com.atlassian.jira.workflow.OSWorkflowManager.createIssue(OSWorkflowManager.java:721)
	... 6 more

And here's my code:

import com.atlassian.jira.ComponentManager
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.config.SubTaskManager
import org.apache.log4j.Logger
import org.apache.log4j.Level

def customFieldManager = ComponentAccessor.getCustomFieldManager()
def constantsManager = ComponentAccessor.getConstantsManager()
def issueFactory = ComponentAccessor.getIssueFactory()
def subTaskManager = ComponentAccessor.getSubTaskManager()
def issueManager = ComponentAccessor.getIssueManager()
def user = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()

MutableIssue issue = (MutableIssue) event.issue
def subTasks = issue.getSubTaskObjects()
def existingTherapies = []
def newTherapies = []

// Functions
def parseTherapySummary(String str) {
 String result = str.substring(str.lastIndexOf(": ") + 2)
 return result
}

// Custom fields
CustomField enrollmentTherapies = customFieldManager.getCustomFieldObjectByName("Enrollment Therapies")
Collection enrollmentTherapiesValue = (Collection)issue.getCustomFieldValue(enrollmentTherapies)

// If no enrolled therapies, exit script and do nothing
if (enrollmentTherapiesValue == null) {
 return
}

// Generate a list of existing therapies
for (Issue st : subTasks) {
 String subtaskSummary = parseTherapySummary(st.summary)
 existingTherapies.add(subtaskSummary)
}

// Check each enrollment therapy see if it already exists
for (String et: enrollmentTherapiesValue) {
 if (!(et in existingTherapies)) {
 newTherapies.add(et)
 }
}

if (newTherapies.size() == 0) {
 return
}

// Create new therapies
newTherapies.each {
   MutableIssue newSubTask = issueFactory.getIssue()
   newSubTask.setSummary((String)it)
   newSubTask.setParentObject(issue)
   newSubTask.setProjectObject(issue.getProjectObject())
   newSubTask.setIssueTypeId(constantsManager.getAllIssueTypeObjects().find{
      it.getName() == "Therapy"
   }.id)

   log.info("New issue ${newSubTask}")
   def newIssueParams = ["Issue" : newSubTask] as Map<String, Object>
   issueManager.createIssueObject(user, newIssueParams)
   subTaskManager.createSubTaskIssueLink(issue, newSubTask, user)
   log.info("Issue with summary ${newSubTask.summary} created.")
}

 Can someone help me understand why this piece of code won't work in the listener?