trigger addContactRole on Opportunity (after Insert, after update) {
    
    List<OpportunityContactRole> newContactRoleList=new List<OpportunityContactRole>();
    List<OpportunityContactRole> oldContactRoleList=new List<OpportunityContactRole>();
    Set<Id> OppId=new Set<Id>();
    Set<Id> ContactId=new Set<Id>();
   
    for(Opportunity oppObj: Trigger.new)
    {
        //Insert condition    
        if(Trigger.isInsert)
        {
            if(oppObj.Investor__c!=null)
            {
                //Creating new contact role
                newContactRoleList.add(new OpportunityContactRole (ContactId=oppObj.Investor__c,OpportunityId=oppObj.Id, Role='Decision Maker',IsPrimary=True));
                                                                                         
            }
        }
        else
        {   
       
                if(oppObj.Investor__c==null && Trigger.oldMap.get(oppObj.Id).Investor__c!=null)
                {
                    //Getting the contact and oppty Id from old values and adding this in set               
                     Opportunity OldoppObj=Trigger.oldMap.get(oppObj.Id);        
                     OppId.add(OldoppObj.id);
                     ContactId.add(OldoppObj.Investor__c);
                    
                }
                else if(oppObj.Investor__c!=null && Trigger.oldMap.get(oppObj.Id).Investor__c==null)
                {
                    //Creating new contact role
                    newContactRoleList.add(new OpportunityContactRole (ContactId=oppObj.Investor__c, OpportunityId=oppObj.Id, Role='Business User',IsPrimary=True));
                }
        }                    
    }
   
   
    try
    {
        //inserting new Contact Roles
        if(newContactRoleList.size()>0) insert newContactRoleList;
       
        //Selecting old Selecting old contact roles
        if ( OppId.size()>0) oldContactRoleList=[Select Id from OpportunityContactRole where ContactId in : ContactId  and OpportunityId in : OppId];
       
        //Deleting old contact roles
        if (oldContactRoleList.size()>0) delete oldContactRoleList;
    }
    catch(Exception e)
    {
        System.debug(e);
        trigger.new[0].addError('Technical error occurred. Please contact to your system administrator or try after some time.');
       
    }
   
   
   
   
}

@isTest(SeeAllData=true)

public class YourTriggerTestClass{
    static testMethod void yourTestMethod(){
    
        //fill all the required field and make sure your data passes the validation rules.
        //insert account
        account acc = new account(
        Name='test account');
        
        insert acc;
        
        //insert contact
        contact ct2 = new Contact(
        LastName='Tester',
        AccountId=acc.id);
        
        insert ct2;
        
        contact ct = new Contact(
        LastName='Tester',
        AccountId=acc.id);
        
        insert ct;
        
        //insert propert
        Property__c p = new Property__c(
        Name='test');
        
        insert p;
        
        //insert campaign
        Campaign camp = new Campaign(
        Name='test camp',
        IsActive=True,
        Property__c=p.Id);
        
        insert camp;
        
        
        //Insert Opportunity
        opportunity opp = new opportunity(
        Name = 'Test oppty',
        
        CampaignId=camp.id,
        StageName='Prospecting',
        CloseDate=system.today(),
        AccountId=acc.id);
        opp.Investor__c=ct.Id
        ;
        
        insert opp;
        
                          
        
        //opp.Investor__c=ct2.id;
        
       // update opp;
        
        opp.Investor__c = null;

        update opp;
        
        }
        }