Issue with required custom field based on the value of a system field

[Arjun Nag]

I am trying to set a custom field as mandatory based on a specific value of the system field  while creating an issue. For other scenarios the custom field should be optional.

For example while creating an issue if the issue type is Defect then a custom field parameter should be mandatory or else it should be optional .

The following script is written on the Script Console of Add-on(ScriptRunner), but it is showing an error as Cannot get property 'null' on null object . Please suggest me some alternative or any modifications required in this code.

import com.onresolve.jira.groovy.user.FieldBehaviours
import groovy.transform.BaseScript

@BaseScript FieldBehaviours fieldBehaviours

def selectedOption = getFieldById(getFieldChanged())
def childField = getFieldById("customfield_13506")

String selectedOptionValue = selectedOption.getValue()

if (selectedOptionValue == "Defect") {
childField.setRequired(true)
} else {
childField.setRequired(false)
}