Getting Error when trying to blank out a Lookup field once a trigger has run to insert Opportunity Team Member.Below is a trigger I wrote to create an Opportunnity Team Member when a lookup field called Secondary Owner is updated. The issue is when I try and blank out the lookup (Say if the secondary owner was input incorrectly) I get the following error. 
 
Below is my trigger

trigger OTOpportunityTrigger on Opportunity (before update) {
    List<OpportunityTeamMember> listOpptyTeamMem = new List<OpportunityTeamMember>();
    
    for(Opportunity oppty : trigger.New){
        OpportunityTeamMember OTM = new OpportunityTeamMember();
        OTM.OpportunityId = oppty.Id;
        OTM.TeamMemberRole = 'Secondary Owner'; 
        OTM.UserId = oppty.Secondary_Owner__c;
        listOpptyTeamMem.add(OTM);   
        
       
            if(listOpptyTeamMem.size() > 0){
        insert listOpptyTeamMem;
       
         
        // get all of the team members' sharing recordsPost to Community
        List<OpportunityShare> shares = [select Id, OpportunityAccessLevel, 
          RowCause from OpportunityShare where OpportunityId = :oppty.Id
          and RowCause = 'Team'];
 
        // set all team members access to read/write
        for (OpportunityShare share : shares) 
          share.OpportunityAccessLevel = 'Edit';
 
        update shares;
    }
}
}

and here is my test class
 

@isTest(SeeAllData=true)
public with sharing class TestOTOpportunityTrigger {

    public static testmethod void test_1() {
             
            //create test account
            Account a  = new Account();
            a.name     = 'Teston';
            insert a;

            //create test opportunity 
            
        Opportunity o  = new Opportunity();
        o.OwnerID = '005i0000004LFZH';
        o.Name = 'Test';
        o.AccountID = '001i000001mCGvY';
        o.Unit_Number__c = 'a01i000000YWLGK';
        o.Financing_Type__c = 'Cash Buyer';
        o.Nature_of_Tenancy__c = 'Severalty';
        o.Sale_Type__c = 'Founder';
        o.CloseDate = Date.today();
        o.stageName = 'S1: Ready to Contract';
        o.Secondary_Owner__c = '005i0000004LFZH';
        
        
        insert o;
        
        OpportunityTeamMember OTM = new OpportunityTeamMember();
        OTM.OpportunityId = o.Id;
        OTM.TeamMemberRole = 'Secondary Owner'; 
        OTM.UserId = o.Secondary_Owner__c;   
        

            
            insert OTM;
             
             
            List<OpportunityTeamMember> OTMs = [SELECT id, UserId, OPPORTUNITYACCESSLEVEL
                                                FROM OpportunityTeamMember
                                                WHERE OpportunityId =: otm.Id
                                                ];
         
             
             
            //update O to execute the trigger
            update otm;
             
            //re-query
            OTMs = [SELECT id, UserId, OPPORTUNITYACCESSLEVEL
                    FROM OpportunityTeamMember
                    WHERE OpportunityId =: otm.Id
                    ];
             

        }

    }