ModalList.cls

public with sharing class ModalList {
    ApexPages.StandardSetController setCon;
    public PageReference init() {
        List<Id> ids = new List<Id>();
        for (SObject i : this.setCon.getSelected()) {
            ids.add((String)i.get('Id'));
        }
        return new PageReference(
                String.format('/lightning/cmp/c__ModalWrapper?c__ids={0}&c__returnUrl={1}', new String[]{
                        String.join(ids, ','),
                        this.setCon.cancel().getUrl()
                })
        );
    }
    public ModalList(ApexPages.StandardSetController controller) {
        this.setCon = controller;
    }
}

ModalList.page

<apex:page
        showHeader="false"
        standardStylesheets="false"
        title="Redirecting"
        standardController="Lead"
        recordSetVar="leads"
        extensions="ModalList"
        cache="false"
        action="{!init}" />