How Do I Using Adaptavist ScriptRunner could remove all linked issues and then add one?

Question

Could someone advise please, how could I delete all linked issue when I change a value of custom filed (for ex. SOW) and then add linked issue mentioned in that field? 
I've tried that code, but it doesn't work at all:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.user.ApplicationUser
import com.onresolve.jira.groovy.user.FieldBehaviours
import com.onresolve.jira.groovy.user.FormField
import groovy.transform.BaseScript
import groovy.transform.Field

@BaseScript FieldBehaviours fieldBehaviours
MutableIssue currentIssue = issue
@Field
String SOW_CF_ID = 'customfield_21900'
FormField sow = getFieldById(SOW_CF_ID)
ApplicationUser currentUser = ComponentAccessor.jiraAuthenticationContext.loggedInUser

if (sow.value) {
    ComponentAccessor.issueLinkManager.removeIssueLinks(issue, currentUser)
}

First of all, because jira doesn't understand what issue it is.

Joshua Yamdogo @ Adaptavist · Accepted Answer

Hello Aleksey,

The problem with your script is that you need to get the underlyingIssue (not just issue) in order to use this in a behaviour. Also, removeIssueLinks() does not accept a MutableIssue as a parameter, so you shouldn't define currentIssue as a MutableIssue.

This is the script that worked for me to remove linked issues:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.user.ApplicationUser
import com.onresolve.jira.groovy.user.FieldBehaviours
import com.onresolve.jira.groovy.user.FormField
import groovy.transform.BaseScript
import groovy.transform.Field

@BaseScript FieldBehaviours fieldBehaviours
def currentIssue = underlyingIssue
@Field
String SOW_CF_ID = 'customfield_21900'
FormField sow = getFieldById(SOW_CF_ID)
ApplicationUser currentUser = ComponentAccessor.jiraAuthenticationContext.loggedInUser

if (sow.value) {
    ComponentAccessor.issueLinkManager.removeIssueLinks(issue, currentUser)
}

I would also say that I really do not think this should be done in a behaviour. A script listener would be a better fit for this use-case. You could set up a listener that fires on an Issue Updated  event. If someone changes the custom field, the listener will delete all issue links once the change to the custom field is made.