Upsert failed.INVALID_ID_FIELD, record id cannot be empty key: []

Question

I'm a beginner developer and I'm building a custom NEW contract button. I'm having issues getting the contract to insert/upsert and I think it has something to do with having a standardsetcontroller in my code. The problem is without it (and recordsetvar), it doesn't allow me to select the visualforce page when creating the list view button.

Error Message

Upsert failed. First exception on row 0 with id 000000000000000; first error: INVALID_ID_FIELD, record id cannot be empty key: [] Error is in expression '{!saveForm}' in component apex:commandButton in page societycontractrequestform: Class.SocietyContractRequest.saveForm: line 65, column 1

Controller Extension

public with sharing class SocietyContractRequest{
        
    public Contract Contract {get;set;}    
          
    public SocietyContractRequest(ApexPages.StandardController controller){
        this.contract = (contract)controller.getRecord();
    }
    
    public SocietyContractRequest(ApexPages.StandardSetController controller){
        this.contract = (contract)controller.getRecord();
    }
    
    //Save page
    public PageReference saveForm(){
        system.debug('Saving Contract...');
        upsert Contract;
        PageReference pageRef = new PageReference('/' + contract.Id);
        pageRef.setRedirect(true);
        return pageRef;
       }
       
    //Cancel and return to the Contract List View
    public PageReference cancelPage(){
        return new ApexPages.Action('{!List}').invoke();
    }
}   

Visualforce Page

<apex:page standardcontroller="Contract" extensions="SocietyContractRequest" recordSetVar="sobjects">
    <Script>
        function setFocusOnLoad() {}
    </Script>    
      <apex:form >        
        <apex:commandButton action="{!saveForm}" value="Save Contract" />        
        <apex:commandButton action="{!cancelPage}" value="Cancel" immediate="true" />                
    </apex:form> 
</apex:page>

Answer 1

What worked for me was creating the list button with the required recordsetvar and StandardSetController... then removing the recordsetvar and changing to using a StandardController after the list button is created. Strange workaround, but it worked.