Attempting to modify a Lead Convert Trigger getting an Attempt to de-reference a null object

We’re using what is basically the example trigger for the lead convert process. We have added a specific queue to the lead process, so if we have auto conversions that come, that have been put in the queue, it errors out. I have tried to specify an ID, or pull the current user ID that’s running the apex which is an integration user. This is my code and I’m getting " response: APEX call failed; received error message: Attempt to de-reference a null object"

I know I shouldn’t hard code a value on this, but I don’t know how to get the running user in the current class, and I’m still not sure that’s causing my error.

@RestResource(urlMapping='/Lead/*')
global with sharing class RestLeadConvert {            

    @HttpPost
    global static String doPost() {
        String result = '';
        String errorMessage = '';
        try{
            RestRequest req = RestContext.request;
            RestResponse res = RestContext.response;
            
            String bodyJson = req.requestBody.toString();
            Map<String, Object> requestContent = (Map<String, Object>)JSON.deserializeUntyped(bodyJson);
            String leadId = (String)requestContent.get('leadId');
            String accountId = (String)requestContent.get('accountId');
            
            Database.LeadConvert lc = new Database.LeadConvert();
            lc.setLeadId(leadId);
            lc.setDoNotCreateOpportunity(True);
            if (accountId != null && accountId != '') {
                lc.setAccountId(accountId);
            }

//Added Conditions// if (String.valueOf(lc.getOwnerId()).startsWith(’00G’)){
// lc.setOwnerId( ‘XXXXXXXXXXXXXXX’);}

            LeadStatus convertStatus = [SELECT Id, MasterLabel FROM LeadStatus WHERE IsConverted=true LIMIT 1];
            lc.setConvertedStatus(convertStatus.MasterLabel);           
            Database.LeadConvertResult lcr ;
                lcr = Database.convertLead(lc);
            system.debug('*****lcr.isSuccess()'+lcr.isSuccess());            
            result = 'success';
        }
        catch(exception ex){
            system.debug('***NOT CONVERTED**');    
            result = 'failed';
            errorMessage = ex.getMessage();       
        }
        
/*        
        JSONGenerator gen = JSON.createGenerator(false);
        gen.writeStartObject();
        gen.writeStringField('result', result);
        gen.writeStringField('errorMessage', errorMessage); 
        gen.writeEndObject();
        
        return gen.getAsString();
*/
        String response = '{'result': '' + result + '', 'errorMessage': '' + errorMessage + ''}';
        return response;
    }   
}

Lead leadRecord = [SELECT OwnerId FROM Lead WHERE Id = :leadId];
if(leadRecord.OwnerId.getSObjectType() == Group.sObjectType) {
  lc.setOwnerId('XXXXXXXXXXXXXXXX');
}