Hi, I have writted a trigger that grabs the Contact ID from a contact related to the opportunity via OpportunityContactRole and inserts this ID in a custom lookup field Opportunity_Contact__c. 
 

trigger UpdateOppConvertContactTrigger on Opportunity (before update) {

    List<OpportunityContactRole> ContactRoles = [SELECT OpportunityId, ContactId
    FROM OpportunityContactRole];

    Map<String, OpportunityContactRole> OppIdToContactMap = new Map<String, OpportunityContactRole>();
    for (OpportunityContactRole ConRole : ContactRoles) {
        OppIdToContactMap.put(ConRole.OpportunityId, ConRole);
    }

    for (Opportunity opp : Trigger.new) {
        if (opp.Opportunity_Contact__c == NULL) {
            OpportunityContactRole newContact = OppIdToContactMap.get(opp.Id);
            opp.Opportunity_Contact__c = newContact.ContactId;
            opp.Type = 'New Customer';
        }
    }
}

Now I am finishing up the work on the test class which just inserts the stuff and then does an update to the Opportunity: 
 

@IsTest
public class UpdateOppConvertContactTriggerTest extends BaseTest {

    @IsTest
    static void testOppInsert() {
            Account testAccount = new Account();
            testAccount.Name = 'name';
        testAccount.BillingStreet = 'Some street';
        testAccount.BillingCity = 'Some City';
        testAccount.Email__c = 'steve@example.com';
        testAccount.Phone = '0123456789';
        testAccount.Industry = 'Agriculture';
        insert testAccount;

        Contact testContact = new Contact();
        testContact.FirstName = 'firstName';
        testContact.LastName = 'lastName';
        testContact.Phone = '0123456789';
        testContact.Email = 'steve@example.com';
        testContact.AccountId = testAccount.Id;
        insert testContact;

            Opportunity testOpportunity = new Opportunity();
            testOpportunity.AccountId = testAccount.Id;
            testOpportunity.Name = 'Test';
            testOpportunity.StageName = 'Appointment Booked';
            testOpportunity.LeadSource = 'Other';
            testOpportunity.Lead_Source_Specific__c = 'Other';
             testOpportunity.CloseDate = Date.newInstance(2018, 12, 09);
            insert testOpportunity;

        OpportunityContactRole testRole = new OpportunityContactRole();
        testRole.OpportunityId = testOpportunity.Id;
        testRole.ContactId = testContact.Id;

        Test.startTest();
        Database.update(testOpportunity);
        Test.stopTest();

    }
}

I continue to get: 
CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, UpdateOppConvertContactTrigger: execution of AfterUpdate
caused by: System.NullPointerException: Attempt to de-reference a null object

No idea why / if error is in test or trigger? 

Much appreciated.