How to set date field if empty in postfuction?

[Jiri]

I am using the following code but that is not now working.

import com.atlassian.jira.component.ComponentAccessor
import java.sql.Timestamp

def customFieldManager = ComponentAccessor.getCustomFieldManager()

def dateCf = customFieldManager.getCustomFieldObject("customfield_20020") // Date time fields require a Timestamp

if (! dateCf.getValue()) {
issue.setCustomFieldValue(dateCf, new Timestamp((new Date()).time))
}

I tried multiple other versions which I found on the internet, but I am not able to figure out.

Thanks for suggestions.