I am getting a System.NullPointerException: Attempt to de-reference a null object
Error is in expression '{!addContact}' in component <apex:commandButton> in page addcontacttojobopportunity: Class.AddContacttoJobOpptController.addContact: line 30, column 1


I am trying to figure out what this means and how to fix it. I want to be able to add multiple existing contacts to a job opportunity (Record type on Opportunities object)  

Heres the code: 

public with sharing class AddContacttoJobOpptController { 
    public Opportunity opp {get;set;}
    public Contact con {get;set;}
    public String selectedConId = '';
   
    public String selectedOppId = '';
  
    String selectedOpportunityID = null;
    
    public AddContacttoJobOpptController(){
 
   
   
        opp = new Opportunity();
    
    
        selectedOppId = Apexpages.currentPage().getParameters().get('aid');
            
        
    }
    public Pagereference addContact(){  
        Set<String> conIdSet = new Set<String>(); 
        selectedConId = Con.Candidate__c ; 
        selectedOppId = Opp.JobOpportunity_Lookup__c ; 
       
        if(selectedConId != null && selectedConId != ''){
            for(Contact con : [select id from Contact where Candidate__c = :selectedOppId]){
                conIdSet.add(con.id);
            }
            if(conIdSet.contains(selectedConId)){
                apexpages.Message msg = new Apexpages.Message(ApexPages.Severity.Info,'The selected contact is already added to the Job Opportunity. Please select another contact to add.');
                apexpages.addmessage(msg);
                return null;
            }
            else{
                Contact con = new Contact(id=selectedConId);
                update con;
                apexpages.Message msg = new Apexpages.Message(ApexPages.Severity.Info,'The selected contact has been added to the Job Opportunity. Now you can select another contact to add.');
                apexpages.addmessage(msg);
                return null;
            }
            
        }
        else{
            apexpages.Message msg = new Apexpages.Message(ApexPages.Severity.Info,'Please select contact then click on Add Contact button.');
            apexpages.addmessage(msg);
            return null;
        }
        
    }
    public Pagereference goBack(){
        return new Pagereference('/'+selectedOppId);
    }

}