I got stuck with the above error:
I also tried def issue = event.issue and no luck. This is on jira version 7.13. Please assist
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.ModifiedValue
import com.atlassian.jira.issue.customfields.option.LazyLoadedOption
import com.atlassian.jira.issue.util.DefaultIssueChangeHolder
import com.atlassian.jira.issue.MutableIssue
Issue issue = event.issue //Error is on this line
def customFieldManager = ComponentAccessor.getCustomFieldManager()
def customField2 = customFieldManager.getCustomFieldObjectByName("System")
LazyLoadedOption systemOption = (LazyLoadedOption) issue.getCustomFieldValue(customField2)
def changeHolder = new DefaultIssueChangeHolder();
def customField4 = customFieldManager.getCustomFieldObjects(issue).find {it.name == "Aquarius Category"}
if (systemOption?.getValue() == "CMS2") {
customField4.updateValue(null, issue, new ModifiedValue(issue.getCustomFieldValue(customField4), ""),changeHolder)
}