Hi all,
After much struggle I had to resort to posting this question in the community. 
This is related to Conga  and I am trying to run a VF page from a List View button in Lightning Experience. Below is the code for Controller, VF Page and the button.

Controller

/**
* @description Builds list of sObject Ids from recordSetVar and constructs the Conductor URL
*/
public class GenericConductorFromListViewController {

    

  private String urlFieldName;
  private String partnerServerUrl;
  private ApexPages.StandardSetController controller;
  private List<Id> recordIds = new List<Id>();
  

  public GenericConductorFromListViewController(ApexPages.StandardSetController controller) {
    this.controller = controller;
    this.urlFieldName = ApexPages.currentPage().getParameters().get('paramURL');
    for (sObject sObj : controller.getSelected()){
      recordIds.add(sObj.Id);
    }
    partnerServerUrl = 'https://'+ApexPages.currentPage().getHeaders().get('Host')+'/services/Soap/u/26.0/'+UserInfo.getOrganizationId();
  } 

  public PageReference prepareConductorUrl() {
    PageReference conductorUrl = new PageReference('https://conductor.congamerge.com?');
    conductorUrl.getParameters().put('MOID', String.join(new List<Id>(recordIds),','));
    conductorUrl.getParameters().put('SessionId',  UserInfo.getSessionID());
    conductorUrl.getParameters().put('ServerUrl',  partnerServerUrl);
    conductorUrl.getParameters().put('UrlFieldName',  urlFieldName);

    return conductorUrl;
  }
}

 

 VF Page
 

<apex:page standardController="Invoice_Number__c" extensions="GenericConductorFromListViewController" recordSetVar="records" >
</apex:page>

Button (Type- List Button ,Display Checkboxes ticked, Contenct Source- URL, Behavior - Display in existing window with sidebar)

/apex/GenericCongaConductorListViewButton?paramURL=Generate_Invoice_URL__c

I am trying to pass a parameter to the Controller's constructor. When I select the button with a record selected, it goes to a blank page and is stuck there. I am not familiar with Apex nor Visualforce and I got to this point scraping knowledge here and there and I am unable go further. Any help is appreciated.

Thanks