<groups name="StandardEventTrigger" labelExpression="Properties" domainClass="gHA_metamodel::StandardEventTrigger" semanticCandidateExpression="var:self">
        <controls xsi:type="properties:LabelDescription" name="SelectedActivityTypes" labelExpression="Selected" valueExpression="service:getTriggerActivityTypes()"/>
        <controls xsi:type="properties:ListDescription" name="ActivityTypeList" labelExpression="activityType" valueExpression="service:getActivityTypes()" displayExpression="var:value">
          <actions labelExpression="Add activity type">
            <initialOperation>
              <firstModelOperations xsi:type="tool_1:SetValue" featureName="activityType" valueExpression="var:selection"/>
            </initialOperation>
          </actions>
        </controls>
      </groups>

public class Services {

public List<String> getActivityTypes(StandardEventTrigger event) {
        List<String> eventTypes = new ArrayList<>();
        for (WEBHOOK_ACTIVITY_TYPES literal : WEBHOOK_ACTIVITY_TYPES.VALUES) {
        	eventTypes.add(literal.toString());
        }
        return eventTypes;
    }
	
	public List<String> getTriggerActivityTypes(StandardEventTrigger event) {
        List<String> eventTypes = new ArrayList<>();
        for (WEBHOOK_ACTIVITY_TYPES literal : event.getActivityType()) {
        	System.out.println("LITERAL: " + literal);
        	eventTypes.add(literal.toString());
        }
        return eventTypes;
    }
}