I'm looking to add an error message to an Opportunity if a user tries to delete it whilst it has any child contracts.

Currently my trigger code is as follows

trigger ClosedOpportunityPreventDeletion on Opportunity (before delete) {

if (system.Trigger.isDelete){

Opportunity[] Opps = [select id, (select id, Opportunity__c from PCFS__r) from Opportunity where id in :Trigger.oldMap.keySet()];

for (Opportunity o : Opps){

if(o.PCFS__r.size()>0){

o.adderror('You cannot delete this Opportunity as it has one or more Customer Forms associated with it');

}

}

}

} 

  Unfortunately, if I use this code, I currently get an error saying "caused by: System.Exception: SObject row does not allow errors".

Any ideas?

With thanks