Why the condition doesn't work in my scriprunner field?

[Ashok Barik]

In my scripted field, I'm getting null instead of condition message. Could someone please have a look and tell me what's wrong?

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.IssueManager;
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.link.IssueLinkManager
import com.atlassian.jira.user.ApplicationUser

Issue issueKey = issue
def id=issueKey.getId()

def customFieldManager = ComponentAccessor.getCustomFieldManager();

def cf = ComponentAccessor.customFieldManager.getCustomFieldObject("customfield_18747")

def field_value = issue.getCustomFieldValue(cf) as Map
def a = field_value?.getAt(null).toString()
def b = field_value?.getAt("1").toString()

if (!b) {
return ("Error in field '" + cf + "': the second part cannot be empty")
}

[Antoine Berry]

Hello @Ashok Barik ,

What is your custom field type ? Are you sure its value is of type Map ?

Can you add 

log.error("field_value  : " + field_value)

log.error("field_value class : " + field_value.getClass())

So we can see the actual value and type ?