Im trying to use a apex script that i have found. But it seems like it does not take care of the problem of opportunities not having a primary contact. If the primary contact is not defined it throws the following error.

Error: Invalid Data.
Review all error messages below to correct your data.
Apex trigger trgMnCopyPrimaryContact caused an unexpected exception, contact your administrator: trgMnCopyPrimaryContact: execution of BeforeUpdate caused by: System.QueryException: List has no rows for assignment to SObject: Trigger.trgMnCopyPrimaryContact: line 6, column 13

trigger trgMnCopyPrimaryContact on Opportunity (before update) {

   for (Opportunity o : Trigger.new) {

       OpportunityContactRole contactRole =
            [select ContactID from OpportunityContactRole where IsPrimary = true and OpportunityId = :o.id];

       if (contactRole != null) {
         o.Opportunity_contact__c = contactRole.ContactID;
       }

   }

 }

Im not a java developer and wonder if someone can help me to add functionality that does not fail if the primary contact is missing.